Well it appears to be documented actually.

Anywhere you'd put an identifier (or chain of identifiers) as part of a variable or subroutine name, you can replace the identifier with a simple scalar variable containing a reference of the correct type - perlref

About identifiers:

If working under the effect of the "use utf8;" pragma, the following rules apply:

        / (?[ ( \p{Word} & \p{XID_Start} ) + [_] ])
          (?[ ( \p{Word} & \p{XID_Continue} ) ]) *    /x
[download]

That is, a "start" character followed by any number of "continue" characters. Perl requires every character in an identifier to also match "\w" (this prevents some problematic cases); and Perl additionally accepts identfier names beginning with an underscore. If not under "use utf8", the source is treated as ASCII + 128 extra controls, and identifiers should match

        / (?aa) (?!\d) \w+ /x
[download]

- perldata, "Identifier parsing"