#!/usr/bin/perl

use strict;
use warnings;

my $tofind="s";
my $replacement="X";
my $variable="snaskes";

print "Before: $variable\n";

$variable =~ s/$tofind[^$tofind]*$/$replacement/;

# OR
# $variable =~ s/$tofind(?![$tofind])$/$replacement/;

print "After: $variable\n";