my @badwords = (
                '31337',
                'hacker'
               );

my @exceptions = (
                  'perl_31337_hacker',
                 );


my $string = 'I am a 31337 hacker. A perl_31337_hacker, that is, not a c_31337_hacker.';

my @occurences = ();

my ($word, $oldword, $badword);

foreach $word (@badwords)
{
    push(@occurences, $string =~ /\b\S*$word\S*\b/g);
}

my %exceptions;
@exceptions{@exceptions} = @exceptions;

foreach $word (@occurences)
{
    if ( !exists ($exceptions{$word}) )
    {
        $oldword = $word;
        
        foreach $badword ( @badwords )
        {
            $word =~ s/$badword/<censored>/g;
        }
        
        $string =~ s/$oldword/$word/;
    }             
}

print $string;

##</code><code>##

I am a <censored> <censored>. A perl_31337_hacker, that is, not a c_<censored>_<censored>.