It seems to me that this was recently posted about on this site.

#!/usr/bin/perl
use strict;

my $find = "r";
my $replace = "X";

my $input = "superfrink";

my $tmp = reverse $input;
$tmp =~ s/$find/$replace/;
my $output = reverse $tmp;

print "input: $input \n";
print "output: $output \n";
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Update: Here is the sample output.

frink@scanbox:~ $ ./397370.pl 
input: superfrink 
output: superfXink 
frink@scanbox:~ $
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