Beginner - Subroutine confusion - perl

Question

I'm a beginner and confused about what's happening inside this Perl subroutine.
I'm using only global variables to simplify things, but it's still not working.
I'm simply trying to print a file's read, write and executable attributes using the file test operators with IF statements.
Can anyone point out the problem for me?
Louie
sub getfileattributes {
if (-r $file) {
$attributes[0] = "readable";
} else { $attributes[0] = "not readable"; }
if (-w _) {
$attributes[1] = "writable";
} else { $attributes[1] = "not writable"; }
if (-x _) {
$attributes[2] = "executable";
} else { $attributes[2] = "not executable"; }
}
my #attributes;
my $file;
foreach $file (#ARGV) {
&getfileattributes;
printf "The file $file is %s, %s and %s\n", #attributes;
}

Answer 1

Using global variables is usually quite bad and points to a design error. In this case, the error seems to be that you don't know how to pass arguments to a sub.
Here is the pattern in Perl:
sub I_take_arguments {
# all my arguments are in #_ array
my ($firstarg, $secondarg, #rest) = #_;
say "1st argument: $firstarg";
say "2nd argument: " .($firstarg+1). " (incremented)";
say "The rest is: [#rest]";
}
Subs are invoked like
I_take_arguments(1, 2, "three", 4);
(Do not invoke them as &nameOfTheSub, this makes use of very special behaviour you don't usually want.)
This would print
1st argument: 1
2nd argument: 3
The rest is: [three 4]
Subroutines can return values, either with the return statement or as the value of the last statement that is executed. These subs are equivalent:
sub foo {return "return value"}
sub bar {"return value"}
I would write your getfileattributes as
sub getFileAttributes {
my ($name) = #_;
return
-r $name ? "readable" : "not readable",
-w $name ? "writable" : "not writable",
-x $name ? "executable" : "not executable";
}
What is happening here? I take an argument $name and then return a list of values. The return keyword could be omitted. The return takes a list of values and does not require parens, so I leave them out. The TEST ? TRUE-STATEMENT : FALSE-STATEMENT operator is known from other languages.
Then, in your loop, the sub would be invoked like
for my $filename (#ARGV) {
my ($r, $w, $x) = getFileAttributes($filename);
say "The file $filename is $r, $w and $x";
}
or
foreach my $file (#ARGV) {
my #attributes = getFileAttributes($file);
printf "The file $file is %s, %s and %s\n", #attributes;
}
Notes:
say is like print, but adds a newline at the end. To use it, you have to have a Perl > 5.10 and you should use 5.010 or whatever version or use feature qw(say).
always use strict; use warnings; unless you know better for sure.
Often, you can write programs without assigning to a variable twice (Single assignment form). This can make reasoning about control flow much easier. This is why global variables (but not global constants) are bad.

Answer 2

You are not actually using global varaibles. My scopes the variables them local to the main routine, so when you call the subroutine, $file and #attributes are scoped to the subroutine, not to the main routine.
Change my to our for $file and #attributes to make the variables global and available to the subroutine.
You can check this for yourself by using the -d argument for perl to run it in the debugger and check the values of the items.