$ perl -MO=Deparse -e 'my $CODE_REF = \"if (\$c == 64) { \$MODE = print \"\\n\"; }" . " else { print chr(\$MODE = \$MODE & 1 ? 32 : 35) x \$c; } ";'
my $CODE_REF = 'SCALAR(0xa00038be0) else { print chr($MODE = $MODE & 1 ? 32 : 35) x $c; } ';
-e syntax OK

##</code><code>##

$ perl -e 'my $CODE_REF = \"if (\$c == 64) { \$MODE = print \"\\n\"; }" . " else { print chr(\$MODE = \$MODE & 1 ? 32 : 35) x \$c; } "; my $ref = ref $code_ref; print $ref ? $ref : "not a ref";'
not a ref

##</code><code>##

$ perl -Mstrict -e 'my $CODE_REF = \"if (\$c == 64) { \$MODE = print \"\\n\"; }" . " else { print chr(\$MODE = \$MODE & 1 ? 32 : 35) x \$c; } "; $CODE_REF->();'
Can't use string ("SCALAR(0xa00038c70) else { print"...) as a subroutine ref while "strict refs" in use at -e line 1.

##</code><code>##

#!/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;
}

##</code><code>##

BEFORE:
$x[X] $y[Y]
AFTER:
$x[A] $y[B]