If I read it right, there'll be a :p5 modifier to say "interpret this regex according to p5 rules". Quote Larry:

So:
    m:w/ foo\ bar \h* \: (baz)*/ really means (expressed in Perl 5 form):
    m:p5/\s*foo bar[\040\t\p{Zs}]*:\s*(baz)*/

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....

Update
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....