#!/usr/bin/perl -w
use strict;

my %bad_words = (
    'crud' => 'dirt',
        'crap' => 'dung',
        'poo' => 'dung',
        'wanker' => 'fool'
);
my $bad_re = join '|', keys %bad_words;

while ( <DATA> ) {
        s/\b($bad_re)\b/$bad_words{lc($1)}/ieg;
        print;
}
__DATA__
"Wanker!" said I.
"Crap," says Travis.
This is horse poo. Poo, I tell you!
"Poo upon all the crud and crap the wanker could see."

##</code><code>##

"fool!" said I.
"dung," says Travis.
This is horse dung. dung, I tell you!
"dung upon all the dirt and dung the fool could see."