If I read it right, there'll be a :p5 modifier to say "interpret this regex according to p5 rules". Quote Larry:
m:w/ foo\ bar \h* \: (baz)*/
really means (expressed in Perl 5 form):
I know which of those forms I'd rather use.
In general, though, I agree with you that it'd be better to convert an entire program at once instead of piecemeal. Unless you lack time and resources....
Larry explicitly says later in the document:
Finally, there's the :p5 modifier, which causes the rest of the regex (or group) to be parsed as a Perl 5 regular expression, including any interpolated strings. (But it still doesn't enable Perl 5's trailing modifiers.)
Should've read more before I typed....