I don't know if this is a big win, but you can do

#Now we build our hashes
foreach(split "",$letters) {
    $letter_hash{$_}++;
}
foreach(split "",$word) {
    $word_hash{$_}++;
}

while(my($key, $value)=each(%word_hash)) {
    # no need to update $letter_hash{$key}
    return 0 if $letter_hash{$key}<$value;
}

return 1;
[download]

This will work even if letters or words ends in a zero character, which would end your loop.
--
Mike