$ cat  /tmp/getpath.pl

#!/usr/bin/perl

use strict;
use warnings;

print GetAbsoluteFileName($0) . "\n";

exit 0;

sub GetAbsoluteFileName {
   my $passed_filename = shift;
   my $absolute_filename;
   use Cwd 'abs_path';

   #expand tildes
   $passed_filename =~ s{ ^ ~ ( [^/]* ) }
                 { $1
                       ? (getpwnam($1))[7]
                       : ( $ENV{HOME} || (getpwuid($<))[7]
                         )
   }ex;

   $absolute_filename = abs_path(ExpandEnvVars( $passed_filename ) );

   return $absolute_filename;
}

#Takes a string and substitutes the value of the environment variable and returns the string back
sub ExpandEnvVars {
    my $env_var = shift;

    # expand variables in $env_var, if environment var does not exist, leave it alone

    $env_var =~ s{
         \$                         # find a literal dollar sign
        (\w+)                       # find a "word" and store it in $1
    }{
        no strict 'refs';           # for $$1 below
        if (exists $ENV{$1}) {
            $ENV{$1};               # expand environment variables only
        } else {
            "\$$1";                 # environment var does not exist, so leave what they gave us
        }
    }egx;

   return $env_var;
}

$ /tmp/getpath.pl
/tmp/getpath.pl

$ ls -1 /tmp/test1/getpath.pl
/tmp/test1/getpath.pl

$ /tmp/test1/getpath.pl
/tmp/test1/getpath.pl

$ export MYTEST=/tmp/test2

$ ls -1 $MYTEST/getpath.pl
/tmp/test2/getpath.pl

$ $MYTEST/getpath.pl
/tmp/test2/getpath.pl