I guess the syntax $$name doesn't "confuse" the parser when name is of the form \w+, while any punctuation variable will look like the variable $$ followed by an operator or other token. $$; is parsed as $$ ;, $$) as $$ ) etc, but _ is not a perl built-in specific character (and is a valid character to start a variable name in every language I can think of) so $$_ is understood like any other $$name variable.