I completely agree with you on the use of white space.

The defined or initialization ($reps{$1} //= 1;) is completely unnecessary code bloat, partly because the values in the hash are only marginally important. For purposes of answering the question "Has this word repeated" $words{$1} = undef; works just as well and the following increment is not required.

The post increment ($words{$1}++;)I should have picked up in the OP. It is mindless cargo culting with no redeeming qualities. Unless logic requires otherwise always pre-increment (++$words{$1};. Pre-increment is never slower (and often faster) than post-increment - which is of trivial importance almost always. Much more important is that sticking the operator out the front makes it easier to be seen making the code easier to understand and maintain.

To my mind the most important aspect of the words hash is that it is a list of words - the keys are more important than the values. The name of the hash tells you that. Especially where a hash is being used to collect unique instances of things, realizing that keys can be the important data and the values are incidental is a liberating break through.