What mr_mischief says, which can be fixed by replacement with $words{$_}++ for split; (no chomp needed). Also, I'd prefer an output loop which didn't construct a potentially long list of keys to iterate. Something like this,

while ($_ = each %words) {
    print $_, ';', $words{$_}, $/;
}
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I like to name my hashes singular for their values, not their keys. That makes the doc-suggested pronounciation work - $count('foo'} is "count of foo" and so on.