# https://perlmonks.org/?node_id=11150731
use strict;
use warnings;

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

print "BEFORE:\n";
print "\$x[$x] \$y[$y]\n";

mod_callers_vars1($x, $y);
print "AFTER1:\n";
print "\$x[$x] \$y[$y]\n";

mod_callers_vars2($x, $y);
print "AFTER2:\n";
print "\$x[$x] \$y[$y]\n";


sub mod_callers_vars1 {
    @_[0,1] = ('A','B');
    return;
}

sub mod_callers_vars2 {
    my ($x_ref, $y_ref) = \(@_);
    $$x_ref = 'C';
    $$y_ref = 'D';
    return;
}

##</code><code>##

BEFORE:
$x[X] $y[Y]
AFTER1:
$x[A] $y[B]
AFTER2:
$x[C] $y[D]