Yet another way to do it. This one is written to stop on the end of sentences (sort of. ;) ). This way phrase don't count if they are accroos a !.? boundary. It probably would have been better to use some modules but it was easy enough to hack together as is. $stop_words should be an array ref with stop words in it.

sub process_keywords {
    my ($text, $weight, $stop_words, $phrase_length, $key_phrases) = @
+_;
    $text =~ s/&[a-z]+;*/ /g;
    $text =~ s/[",']//g;
    $text =~ s/[.!?]/ . /g;
    $text =~ s/\s+/ /gs;
    my @words = map { lc($_) } split(' ', $text);
    @words = grep { length($_) } @words;
    my $stops = { map { $_ => 1 } @$stop_words };

    for my $word (0 .. scalar @words - 1) {
       next if exists $stops->{$words[$word]};
       for my $length (0 .. $phrase_length) {
          next unless defined @words[$word + $length];
          next if exists $stops->{$words[$word+$length]};
          my $phrase = join(' ', @words[$word .. $word+$length]);
          $key_phrases->{$phrase} += $weight;
       }
    }
    return $key_phrases;
}
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