The '//' and '//=' (defined or) operators aren't in the core because they tend to be ambigious with an empty regex (like in split //, $str). They should be in 5.10, though.

Ehm, I have no idea what you're responding to, but I do think your example isn't a very good one. The comma in split //, $foo would disambiguate. Perl knows when to expect a value and when to expect an operator (I'm sure you've seen those "... found where ... expected" syntax errors).