#!/usr/bin/env perl

use strict;
use warnings;

my ($x, $y) = qw{X Y};

print "BEFORE:\n";
print "\$x[$x] \$y[$y]\n";
mod_callers_vars(\$x, \$y);
print "AFTER:\n";
print "\$x[$x] \$y[$y]\n";

sub mod_callers_vars {
    my ($x_ref, $y_ref) = @_;
    $$x_ref = 'A';
    $$y_ref = 'B';
    return;
}