Here's another approach that's not necessarily better, but has a certain charm anyway:

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

@ARGV = qw(/usr/dict/words);

my $longest = '';

while (<>) {
    chomp;
    next unless length > length $longest;
    if (length == tr/qwertasdfgzxcvb/qwertasdfgzxcvb/) {
        $longest = $_;
    }
}

print $longest;
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