Using a var to select a var in Perl - perl

Question

I thought you used to be able to do this in "strict" mode, but I may be remembering incorrectly. Is is possible for something like this to work...
use strict;
use warnings;
package SomePackage;
my $TargetPID="demo:5"; #using "our" also works, but not in strict mode
my $VarName="TargetPID";
print ${$VarName}; #works but not in strict or if the var is declared with "my"
exit;
The reason I'm interested is that I'm trying to select a variable based on a text flag in a text file and I'd like to read in the content of the text file into a hash, then substitute some identifier along the lines of "#TargetPID#" with the corresponding variable. Being also able to specify both a package and a variable (or constant) would be a nice tbonus.

Answer 1

I can't think of any advantage of doing it the way you are trying to over just using a hash:
use strict;
use warnings;
package SomePackage;
my %vars = ();
$vars{'TargetPID'}="demo:5";
my $VarName="TargetPID";
print $vars{$VarName};
exit;
If you really must use $TargetPID as a variable and not a member of a hash, you can use eval:
my $TargetPID = "demo:5";
my $VarName = '$TargetPID';
print eval $VarName;
Or, if for some reason you need the value of $VarName to be 'TargetPID' and not '$TargetPID', you can do print eval '$' . $VarName.

Answer 2

You can use the PadWalker module for this. From the documentation:
PadWalker is a module which allows you to inspect (and even change!)
lexical variables in any subroutine which called you. It will only
show those variables which are in scope at the point of the call.
In your case, you would need to use peek_my, which does what it says: it allows you to peek into variables declared by my in a given scope.
#!/usr/bin/perl
use warnings;
use strict;
package SomePackage;
use PadWalker qw/peek_my/;
my $TargetPID = "demo:5";
my $VarName = "TargetPID";
print ${peek_my(0)->{'$' . $VarName}}
The subroutine peek_my takes one argument, a level, which is the number of subroutine calls to go back on the stack. It then returns a hash map of all the lexical my variables that were in scope at the time of the given call. In your case, the variable you want is defined in the same scope as where it is needed, so you would pass in 0, to go back 0 subroutine calls. Then you pull out the data you need like any other hash ref.
Be careful though, from the documentation:
PadWalker is particularly useful for debugging (emphasis mine.) It's even used by
Perl's built-in debugger. (It can also be used for evil, of course.)
I wouldn't recommend using PadWalker directly in production code, but
it's your call. Some of the modules that use PadWalker internally are
certainly safe for and useful in production.