So the question that left me with was, 'what exactly is considered a punctuation character?'

The first two paragraphs of perlvar say, in part:

Variable names in Perl can have several formats.  Usually, they
must begin with a letter or underscore ...
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Perl variable names may also be a sequence of digits, a single
punctuation character, or ...
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I think its unambiguous from that opening that an underscore is grouped with letters rather than punctuation characters.

Dave.