I think you want to do the hash the other way around. I.e. make it a hash of lengths and push each word onto an anonymous array of words of than length:

sub longest_words {
    my %W;
    push (@{$W{length($_)}}, $_) for (split /\s+/, join ' ', @_);
    return @{$W{ (sort{$b <=>$a} keys %W)[0]} };
}
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Dingus