My program creates an object, which, in turn, creates another object
MainScript.pm
use module::FirstModule qw ($hFirstModule);
$hFirstModule->new(parametres);
$hFirstModule->function();
FirstModule.pm
use Exporter ();
#EXPORT = qw($hFirstModule);
use module::SecondModule qw ($hSecondModule);
sub new {
my $className = shift;
my $self = { key => 'val' };
bless $self, $classname;
return $self;
}
sub function{
$hSecondModule->new(parametres);
#some other code here
}
I want to acces $hSecondModule from MainScript.pm.
It depends.
We would have to see the actual code. What you've shown is a bit ambiguous. However, there are two scenarios.
You can't
If your code is not exactly like what you have shown as pseudo-code, then there is no chance to do that. Consider this code in &module1::function.
sub function {
my $obj = Module2->new;
# ... more stuff
return;
}
In this case, you are not returning anything, and the $obj is lexically scoped. A lexical scope means that it only exists inside of the closest {} block (and all blocks inside that). That's the block of the function sub. Once the program returns out of that sub, the variable goes out of scope and the object is destroyed. There is no way to get to it afterwards. It's gone.
Even if it was not destroyed, you cannot reach into a different scope.
You can
If you however return the object from the function, then you'd have to assign it in your script, and then you can access it later. If the code is exactly what you've shown above, this works.
sub function {
my $obj = Module2->new;
# nothing here
}
In Perl, subs always return the last true statement. If you don't have a return and the last statement is the Module2->new call, then the result of that statement, which is the object, is returned. Of course it also works if you actually return explicitly.
sub function {
return Module2->new;
}
So if you assign that to a variable in your script, you can access it in the script.
my $obj = module1->function();
This is similar to the factory pattern.
This is vague, but without more information it's impossible to answer the question more precicely.
Here is a very hacky approach that takes your updated code into consideration. It uses Sub::Override to grab the return value of the constructor call to your SecondModule thingy. This is something that you'd usually maybe do in a unit test, but not in production code. However, it should work. Here's an example.
Foo.pm
package Foo;
use Bar;
sub new {
return bless {}, $_[0];
}
sub frobnicate {
Bar->new;
return;
}
Bar.pm
package Bar;
sub new {
return bless {}, $_[0];
}
sub drink {
return 42; # because.
}
script.pl
package main;
use Foo; # this will load Bar at compile time
use Sub::Override;
my $foo = Foo->new;
my $bar; # this is lexical to the main script, so we can use it inside
my $orig = \&Bar::new; # grab the original function
my $sub = Sub::Override->new(
"Bar::new" => sub {
my $self = shift;
# call the constructor of $hSecondModule, grab the RV and assign
# it to our var from the main script
$bar = $self->$orig(#_);
return $bar;
}
);
$foo->frobnicate;
# restore the original sub
$sub->restore;
# $bar is now assigend
print $bar->drink;
Again, I would not do this in production code.
Let's take a look at the main function. It first creates a new Foo object. Then it grabs a reference to the Bar::new function. We need that as the original, so we can call it to create the object. Then we use Sub::Override to temporarily replace the Bar::new with our sub that calls the original, but takes the return value (which is the object) and assigns it to our variable that's lexical to the main script. Then we return it.
This function will now be called when $foo->frobnicate calls Bar->new. After that call, $bar is populated in our main script. Then we restore Bar::new so we don't accidentally overwrite our $bar in case that gets called again from somewhere else.
Afterwards, we can use $bar.
Note that this is advanced. I'll say again that I would not use this kind of hack in production code. There is probably a better way to do what you want. There might be an x/y problem here and you need to better explain why you need to do this so we can find a less crazy solution.
Related
I have this perl object. After the object is instantiated, I'm trying to add a new method to the object within a loader method, that can then be called later.
I've tried a whole bunch of stuff that hasn't worked. Examples include:
sub loader {
my ($self) = #_;
sub add_me {
my ($self, $rec) = #_
warn "yayyyyyy";
return $rec;
}
#here are the things I've tried that dont work:
# &{$self->{add_me}} = \&add_me;
# \&{$self->{add_me}} = \&add_me;
# assuming the class definition is in Holder::Class try to add it to symblol table
# *{Holder::Class::add_me} = \&add_me;
}
EDIT:
The reason that I need to do this is I'm adding a hook in my code where the user of my software will have the ability to inject their own sub to edit a data structure as they will.
To do this, they will be able to edit a secondary file that will only contain one sub and get the data structure in question passed in, so something like:
sub inject_a_sub {
my ($self, $rec) = #_;
#do stuff to $rec
return $rec;
}
then inside my original object upon its instantiation, I check to see if the above mentioned file exists, and if so read its contents and eval them. Lastly, I want to make the eval'd code which is just a sub, a method of my object. To be precise, my object is already inheriting a method called do_something and i want to make the sub read in by the eval override the do_something method being inherited so that when called the sub from the external file runs.
its a weird problem :/
and it hurts me :(
Obi wan kenobi you're my only hope!
Cheers!
If you just want to attach functionality to a specific object, and don't need inheritance, you can store a code ref in the object and call it.
# Store the code in the object, putting it in its own
# nested hash to reduce the chance of collisions.
$obj->{__actions}{something} = sub { ... };
# Run the code
my #stuff = $obj->{__actions}{something}->(#args);
Problem is, you need to check that $obj->{__actions}{something} contains a code reference. What I would suggest is to wrap a method around this procedure.
sub add_action {
my($self, $action, $code) = #_;
$self->{__actions}{$action} = $code;
return;
}
sub take_action {
my($self, $action, $args) = #_;
my $code = $self->{__actions}{$action};
return if !$code or ref $code ne 'CODE';
return $code->(#$args);
}
$obj->add_action( "something", sub { ... } );
$obj->take_action( "something", \#args );
If you already know the class name you want to inject a method into, write the subroutine as normal but use the fully qualified name.
sub Some::Class::new_method {
my $self = shift;
...
}
Note that any globals inside that subroutine will be in the surrounding package, not in Some::Class. If you want persistent variables use state inside the subroutine or my outside the subroutine.
If you don't know the name at compile time, you'll have to inject the subroutine into the symbol table, so you were close with that last one.
sub inject_method {
my($object, $method_name, $code_ref) = #_;
# Get the class of the object
my $class = ref $object;
{
# We need to use symbolic references.
no strict 'refs';
# Shove the code reference into the class' symbol table.
*{$class.'::'.$method_name} = $code_ref;
}
return;
}
inject_method($obj, "new_method", sub { ... });
Methods in Perl are associated with a class, not an object. In order to assign a method to a single object, you have to put that object into its own class. Similar to the above, but you have to create a subclass for every instance.
my $instance_class = "_SPECIAL_INSTANCE_CLASS_";
my $instance_class_increment = "AAAAAAAAAAAAAAAAAA";
sub inject_method_into_instance {
my($object, $method_name, $code_ref) = #_;
# Get the class of the object
my $old_class = ref $object;
# Get the special instance class and increment it.
# Yes, incrementing works on strings.
my $new_class = $instance_class . '::' . $instance_class_increment++;
{
# We need to use symbolic references.
no strict 'refs';
# Create its own subclass
#{$new_class.'::ISA'} = ($old_class);
# Shove the code reference into the class' symbol table.
*{$new_class.'::'.$method_name} = $code_ref;
# Rebless the object to its own subclass
bless $object, $new_class;
}
return;
}
I left out the code to check whether or not the instance has already had this treatment by checking if its class matches /^${instance_class}::/. I leave that as an exercise for you. Creating a new class for every object is not cheap and will cost memory.
There are valid reasons to do this, but they are exceptional. You should really, really question whether you should be doing this sort of monkey patching. In general, action at a distance should be avoided.
Can you accomplish the same thing using a subclass, delegation or role?
There already exist Perl OO systems which will do this for you and much much more. You should be using one. Moose, Moo (via Role::Tiny) and Mouse can all add roles to an instance.
I looked up a few answers dealing with this warning, but neither did they help me, nor do I truly understand what Perl is doing here at all. Here's what I WANT it to do:
sub outerSub {
my $dom = someBigDOM;
...
my $otherVar = innerSub();
return $otherVar;
sub innerSub {
my $resultVar = doStuffWith($dom);
return $resultVar;
}
}
So basically, I have a big DOM object stored in $dom that I don't want to pass along on the stack if possible. In outerSub, stuff is happening that needs the results from innerSub. innerSub needs access to $dom. When I do this, I get this warning "Variable $dom will not stay shared".
What I don't understand:
Does this warning concern me here? Will my intended logic work here or will there be strange things happening?
If it doesn't work as intended: is it possible to do that? To make a local var visible to a nested sub? Or is it better to just pass it as a parameter? Or is it better to declare an "our" variable?
If I push it as a parameter, will the whole object with all its data (may have several MB) be pushed on the stack? Or can I just pass something like a reference? Or is Perl handling that parameter as a reference all by itself?
In "Variable $foo will not stay shared" Warning/Error in Perl While Calling Subroutine, someone talks about an anonymous sub that will make this possible. I did not understand how that works, never used anything like that.
I do not understand that explanation at all (maybe cause English is not my first language): "When the inner subroutine is called, it will see the value of the outer subroutine's variable as it was before and during the first call to the outer subroutine; in this case, after the first call to the outer subroutine is complete, the inner and outer subroutines will no longer share a common value for the variable.":
What does "the first call to the outer subroutine is complete? mean"
I mean: first I call the outer sub. The outer sub calls the inner sub. The outer sub is of course still running. Once the outer sub is complete, the inner sub will be finished as well. Then how does any of this still apply when the inner sub is already finished? And what about the "first" call? When is the "second" call happening... sorry, this explanation confuses me to no end.
Sorry for the many questions. Maybe someone can at least answer some of them.
In brief, the second and later times outerSub is called will have a different $dom variable than the one used by innerSub. You can fix this by doing this:
{
my $dom;
sub outerSub {
$dom = ...
... innerSub() ...
}
sub innerSub {
...
}
}
or by doing this:
sub outerSub {
my $dom = ...
*innerSub = sub {
...
};
... innerSub() ...
}
or this:
sub outerSub {
my $dom = ...
my $innerSub = sub {
...
};
... $innerSub->() ...
}
All the variables are originally preallocated, and innerSub and outerSub share the same $dom. When you leave a scope, perl goes through the lexical variables that were declared in the scope and reinitializes them. So at the point that the first call to outerSub is completed, it gets a new $dom. Because named subs are global things, though, innerSub isn't affected by this, and keeps referring to the old $dom. So if outerSub is called a second time, its $dom and innerSub's $dom are in fact separate variables.
So either moving the declaration out of outerSub or using an anonymous sub (which gets freshly bound to the lexical environment at runtime) fixed the problem.
You need to have an anonymous subroutine to capture variables:
my $innerSub = sub {
my $resultVar = doStuffWith($dom);
return $resultVar;
};
Example:
sub test {
my $s = shift;
my $f = sub {
return $s x 2;
};
print $f->(), "\n";
$s = "543";
print $f->(), "\n";
}
test("a1b");
Gives:
a1ba1b
543543
If you want to minimize the amount of size passing parameters to subs, use Perl references. The drawback / feature is that the sub could change the referenced param contents.
my $dom = someBigDOM;
my $resultVar = doStuffWith(\$dom);
sub doStuffWith {
my $dom_reference = shift;
my $dom_contents = $$dom_reference;
#...
}
Following http://www.foo.be/docs/perl/cookbook/ch10_17.htm , you should define a local GLOB as follows :
local *innerSub = sub {
...
}
#You can call this sub without ->
innerSub( ... )
Note that even if warning is displayed, the result stay the same as it should be expected : variables that are not defined in the inner sub are modified in the outer sub scope. I cannot see what this warning is about.
I'm writing Perl for quite some time now and always discovering new things, and I just ran into something interesting that I don't have the explanation to it, nor found it over the web.
sub a {
sub b {
print "In B\n";
}
}
b();
how come I can call b() from outside its scope and it works?
I know its a bad practice to do it, and I dont do it, I use closured and such for these cases, but just saw that.
Subroutines are stored in a global namespace at compile time. In your example b(); is short hand for main::b();. To limit visibility of a function to a scope you need to assign an anonymous subroutines to a variable.
Both named and anonymous subroutines can form closures, but since named subroutines are only compiled once if you nest them they don't behave as many people expect.
use warnings;
sub one {
my $var = shift;
sub two {
print "var: $var\n";
}
}
one("test");
two();
one("fail");
two();
__END__
output:
Variable "$var" will not stay shared at -e line 5.
var: test
var: test
Nesting named subroutines is allowed in Perl but it's almost certainly a sign that the code is doing someting incorrectly.
The "official" way to create nested subroutines in perl is to use the local keyword. For example:
sub a {
local *b = sub {
return 123;
};
return b(); # Works as expected
}
b(); # Error: "Undefined subroutine &main::b called at ..."
The perldoc page perlref has this example:
sub outer {
my $x = $_[0] + 35;
local *inner = sub { return $x * 19 };
return $x + inner();
}
"This has the interesting effect of creating a function local to another function, something not normally supported in Perl."
The following prints 123.
sub a {
$b = 123;
}
a();
print $b, "\n";
So why are you surprised that the following does too?
sub a {
sub b { return 123; }
}
a();
print b(), "\n";
Nowhere is any request for $b or &b to be lexical. In fact, you can't ask for &b to be lexical (yet).
sub b { ... }
is basically
BEGIN { *b = sub { ... }; }
where *b is the symbol table entry for $b, #b, ..., and of course &b. That means subs belong to packages, and thus can be called from anywhere within the package, or anywhere at all if their fully qualified name is used (MyPackage::b()).
Subroutines are defined during compile time, and are not affected by scope. In other words, they cannot truly be nested. At least not as far as their own scope is concerned. After being defined, they are effectively removed from the source code.
Suppose I have a function that returns a closure:
sub generator
{
my $resource = get_resource();
my $do_thing = sub
{
$resource->do_something();
}
return $do_thing;
}
# new lexical scope
{
my $make_something_happen = generator();
&$make_something_happen();
}
I would like to be able to ensure that when $make_something_happen is removed from scope, I am able to call some $resource->cleanup();
Of course, if I had a class, I could do this with a destructor, but that seems a bit heavyweight for what I want to do. This isn't really an "object" in the sense of modelling an object, it's just a functiony thing that needs to execute some code on startup and immediately prior to death.
How would I do this in Perl( and, out of curiosity, does any language support this idea)?
I'd just use a class for this. Bless the subroutine reference and still use it like you are. The get_resource then uses this class. Since I don't know what that looks like, I'll leave it up to you to integrate it:
package Gozer {
sub new {
my( $class, $subref );
bless $subref, $class;
}
sub DESTROY {
...; #cleanup
}
}
If every thing can have it's own cleanup, I'd use the class to group two code refs:
package Gozer {
sub new {
my( $class, $subref, $cleanup );
bless { run => $subref, cleanup => $cleanup }, $class;
}
sub DESTROY {
$_[0]{cleanup}();
}
}
In Perl, I don't think this is heavyweight. The object system simply attaches labels to references. Not every object needs to model something, so this is a perfectly fine sort of object.
It would be nice to have some sort of finalizers on ordinary variables, but I think those would end up being the same thing, topologically. You could do it with Perl as a tie, but that's just an object again.
I think I understand your question. In my case I want:
* A global variable that may be set at any point during the script's runtime
* To last right up to the end of the life of the script
* Explicitly clean it up.
It looks like I can do this by defining an END block; It will be run "as late as possible".
You should be able to do your $resource->cleanup(); up in there.
More here:
http://perldoc.perl.org/perlmod.html#BEGIN%2c-UNITCHECK%2c-CHECK%2c-INIT-and-END
The begincheck program on that page has the code.
In script i initialize several handlers and set variables which should be available for functions in separate modules. Which is the best way to use them ($q, $dbh, %conf) in modules?
Example pseudo module:
package My::Module
sub SomeFunction (
#data = $dbh->selectrow_array("SELECT * FROM Foo WHERE Bar = ?", undef, $conf{Bar} );
return $q->p( "#data" );
)
1;
Example pseudo script:
use CGI;
use DBI;
use My::Module;
our $q = new CGI;
our $dbh = some_connenction_function($dsn);
our %conf = ( Foo => 1, Bar => 2, Random => some_dynamic_data() );
I understand that using main:: namespace will work, but there sholud be cleaner way? Or not?
package My::Module
Your modules should be independent of the context. That is, they shouldn't expect that $dbh is the database handler, or that they should return stuff in $q. or that configuration is kept in %conf.
For example, what if you suddenly find yourself with two instances of database handles? What do you do? I hate it when a module requires me to use module specific variables for configuration because it means I can't use two different instances of that module.
So you have two choices:
Either pass in the required data each time.
Create an object instance (that's right Object Oriented Programming) to store the required information.
Let's look at the first instance using your pseudo code:
sub someFunction (
%param = #_;
%conf = %{param{conf};
#data = $param{dbh}->selectrow_array(
"SELECT * FROM Foo WHERE Bar = ?", undef, $conf{Bar}
);
return $param{q}->p( "#data" );
)
1;
Example pseudo script:
use CGI;
use DBI;
use My::Module;
my $q = new CGI;
my $dbh = some_connenction_function($dsn);
my %conf = ( Foo => 1, Bar => 2, Random => some_dynamic_data() );
someFunction (
q => $q,
dbh => $dbh,
conf => \%conf,
);
Here I'm using parameter calls. Not too bad. Is it? Now if you need another select statement, you can use different variables.
Sure, but what if you don't want to keep passing variables all of the time. Well then, you can use an Object Oriented Techniques. Now, relax and calm down. There are many, many good reasons to use object oriented design:
It can simplify your programming: This confuses people because object oriented programming means thinking about how your program will work, then designing it, then creating the various objects. All you want to do is code and get it out of the way. The truth is that by thinking about your program and designing it makes your program work better, and you code faster. The design aspect keeps the complexity out of your main code and tucks it safely away in small, easily digestible routines.
It's what is cool in Perl: You're going to have to get use to object oriented Perl because that's what everyone else is writing. You'll be seeing lots of my $foo = Foo::Bar->new; type of statements. Newer Perl modules only have object oriented interfaces, and you won't be able to use them.
Chicks dig it: Hey, I'm just grasping at straws here...
Let's see how an object oriented approach might work. First, let's see the main program:
use CGI;
use DBI;
use My::Module;
my $q = new CGI;
my $dbh = some_connenction_function($dsn);
my %conf = ( Foo => 1, Bar => 2, Random => some_dynamic_data() );
my $mod_handle = My::Module->new (
q => $q,
dbh => $dbh,
conf => \%conf,
);
$mod_handle->someFunction;
In the above, I now create an object instance that contains these variables. And, magically, I've changed your Functions into Methods. A method is simply a function in your Class (aka module). The trick is that my instance (the variable $mod_handler has all of your required variables stored away nice and neat for you. The $mod_hander-> syntax merely passes this information for my into my functions I mean methods.
So, what does your module now look like? Let's look at the first part where I have the Constructor which is simply the function that creates the storage for my variables I need:
sub new {
my $class = shift;
my %param = #_;
my $self = {};
bless $self, $class
$self->{Q} = $q;
$self->{DBH} = $dbh;
$self->{CONF} = $conf;
return $self;
}
Let's look at the first thing that is a bit different: my $class = shift;. Where is this coming from? When I call a function with the syntax Foo->Bar, I am passing Foo as the first parameter in the function Bar. Thus, $class is equal to My::Module. It is the same as if I called your function this way:
my $mod_handle = My::Module::new("My::Module", %params);
instead of:
my $mod_handle = My::Module->new(%params);
The next thing is the my $self = {}; line. This is creating a reference to a hash. If you don't understand references, you should look at Mark's Reference Tutorial that's included in Perldocs. Basically, a reference is the memory location where data is stored. In this case, my hash doesn't have a name, all I have is a reference to the memory where it's stored called $self. In Perl, there's nothing special about the name new or $self, but they're standards that everyone pretty much follows.
The bless command is taking my reference, $self, and declaring it a type My::Module. That way, Perl can track whether $mod_handle is the type of instance that has access to these functions.
As you can see, the $self reference contains all the variables that my functions need. And, I conveniently pass this back to my main program where I store it in $mod_handle.
Now, let's look at my Methods:
sub SomeFunction {
$self = shift;
my $dbh = $self->{DBH};
my $q = $self->{Q};
my %conf = %{self->{CONF}};
#data = $dbh->selectrow_array(
"SELECT * FROM Foo WHERE Bar = ?", undef, $conf{Bar}
);
return $param{q}->p( "#data" );
}
Again, that $self = shift; line. Remember I'm calling this as:
$mod_handle->SomeFunction;
Which is the same as calling it:
My::Module::SomeFunction($mod_handle);
Thus, the value of $self is the hash reference I stored in $mod_handle. And, that hash reference contains the three values I am always passing to this function.
Conclusion
You should never share variables between your main program and your module. Otherwise, you're stuck not only with the same name in your program each and every time, but you must be careful not to use your module in parallel in another part of your program.
By using object oriented code, you can store variables you need in a single instance and pass them back and forth between functions in the same instance. Now, you can call the variables in your program whatever you want, and you can use your module in parallel instances. It improves your program and your programming skills.
Beside, you might as well get use to object oriented programming because it isn't going away. It works too well. Entire languages are designed to be exclusively object oriented, and if you don't understand how it works, you'll never improve your skills.
And, did I mention chicks dig it?
Adium
Before all the Perl hackers descend upon me. I want to mention that my Perl object oriented design is very bad. It's way better than what you wanted, but there is a serious flaw in it: I've exposed the design of my object to all the methods in my class. That means if I change the way I store my data, I'll have to go through my entire module to search and replace it.
I did it this way to keep it simple, and make it a bit more obvious what I was doing. However, as any good object oriented programmer will tell you (and second rate hacks like me) is that you should use setter/getter functions to set your member values.
A setter function is pretty simple. The template looks like this:
sub My_Method {
my $self = shift;
my $value = shift;
# Something here to verify $value is valid
if (defined $value) {
$self->{VALUE} = $value;
}
return $self->{VALUE};
}
If I call $instance->My_Method("This is my value"); in my program, it will set $self->{VALUE} to This is my value. At the same time, it returns the value of $self->{VALUE}.
Now, let's say I all it this way:
my $value = $instance->My_Method;
My parameter, $value is undefined, so I don't set the value $self->{VALUE}. However, I still return the value anyway.
Thus, I can use that same method to set and get my value.
Let's look at my Constructor (which is a fancy name for that new function):
sub new {
my $class = shift;
my %param = #_;
my $self = {};
bless $self, $class
$self->{Q} = $q;
$self->{DBH} = $dbh;
$self->{CONF} = $conf;
return $self;
}
Instead of setting the $self->{} hash reference directly in this program, good design said I should have used getter/setter functions like this:
sub new {
my $class = shift;
my %param = #_;
my $self = {};
bless $self, $class
$self->Q = $q; #This line changed
$self->Dbh = $dbh; #This line changed
$self->Conf = $conf; #This line changed
return $self;
}
Now, I'll have to define these three subroutines, Q, Dbh, and Conf, but now my SomeFunction method goes from this:
sub SomeFunction {
$self = shift;
my $dbh = $self->{DBH};
my $q = $self->{Q};
my %conf = %{self->{CONF}};
#data = $dbh->selectrow_array(
"SELECT * FROM Foo WHERE Bar = ?", undef, $conf{Bar}
);
return $param{q}->p( "#data" );
}
To this:
sub SomeFunction {
$self = shift;
my $dbh = $self->Dbh; #This line changed
my $q = $self->Q; #This line changed
my %conf = %{self->Conf}; #This line changed
#data = $dbh->selectrow_array(
"SELECT * FROM Foo WHERE Bar = ?", undef, $conf{Bar}
);
return $param{q}->p( "#data" );
}
The changes are subtle, but important. Now my new function and my SomeFunction have no idea how these parameters are stored. The only place that knows how they're stored is the getter/setter function itself. If I change the class data structure, I don't have to modify anything but the getter/setter functions themselves.
Postscript
Here's food for thought...
If all of your SQL calls are in your My::Module function, why not simply initialize the $dbh, and the $q there in the first place. This way, you don't need to include the Use Dbi; module in your program itself. In fact, your program now remains blissfully ignorant exactly how the data is stored. You don't know if it's a SQL database, a Mongo database, or even some flat Berkeley styled db structure.
I'm including a module I did for a job long, long ago where I tried to simplify the way we use our database. This module did several things:
It handled everything about the database. The initialization, and all the handles. Thus, your main program didn't have to use any module but this.
It also moved away from the developer writing select statements. Instead, you defined what fields you wanted, and it figured out how to do the query for you.
It returns an incriminator function that's used to fetch the next row from the database.
Take a look at it. It's not the greatest, but you'll see how I use object oriented design to remove all of the various issues you are having. To see the documentation, simply type in perldoc MFX:Cmdata on the command line.
Using main:: explicitly is perfectly clean
As an alternative, you can pass that data into the module constructor method assuming your module is object based (or copy it into module's own variables from main:: in the constructor). This way the not-quite-perfwectly-pretty main:: is hidden away from the rest of the module.
One cleaner way is to use Exporter to make symbols from other namespaces available in your package. If you are just an occasional Perl programmer, I wouldn't bother with it because there's a bit of a learning curve to it, there are lots of gotchas, and for a toy project it makes the code even more complex. But it is essential for larger projects and for understanding other people's modules.
Usually, there are global variables (or especially global functions) in a package that you want to use in a different module without qualifying it (i.e., prepending package:: to every variable and function call). Here's one way that Exporter might be used to do that:
# MyModule.pm
package MyModule;
use strict; use warnings;
use Exporter;
our #EXPORT = qw($variable function);
our $variable = 42; # we want this var to be used somewhere else
sub function { return 19 }; # and want to call this function
...
1;
# my_script.pl
use MyModule;
$variable = 16; # refers to $MyModule::variable
my $f = function(); # refers to &MyModule::function
Your problem spec is backwards (not that there's necessarily anything wrong with that) -- you want variables in the main script and main package to be visible inside another module. For a short list of variables/functions that are used many times, a cleaner way to do that might be to hack at the symbol table:
package My::Module;
# required if you use strict . You do use strict , don't you?
our ($q, $dbh, %conf);
*q = \$main::q;
*dbh = \$main::dbh;
*conf = \%main::conf;
...
Now $My::Module::q and $main::q refer to the same variable, and you can just use $q in either the main or the My::Module namespace.