According to perlvar,

Variable names in Perl can have several formats. Usually, they must begin with a letter or underscore ... and may contain letters, digits, underscores...

Perl variable names may also be a sequence of digits or a single punctuation or control character. ...

Finally, new in Perl 5.6, Perl variable names may be alphanumeric strings that begin with control characters (or better yet, a caret). ...

The term punctuation variable refers to the second variation of the second paragraph. Consider it short for punctionuation-based variable, using the noun implicitly as an adjective.

What characters constitute the set of punctuation characters? It doesn't say, other than by implication all the ones with built-in meanings. I've seen the British currency symbol used in golf. Perhaps someone familiar with the source code will jump in and say what exactly is considered punctuation.

—John