*PI = \3.14159;
*STR = \'Some string';
# etc.

##</code><code>##

use warnings;
use strict;
no strict qw/vars/;

*STR = \'Some string';
my ($str1, $str2) = (\'Some string', $STR);

sub foo(\$) { return }

eval { foo($STR) };
warn "$@";
eval { foo($str1) };
warn "$@";
eval { foo($str2) };
warn "$@";

##</code><code>##

$ perl tst.pl
Warning: something's wrong at tst.pl line 10.
Warning: something's wrong at tst.pl line 12.
Warning: something's wrong at tst.pl line 14.