[P5P] [perl #128225] substitution within (?{}) causes segmentation fault

On Mon May 30 05:08:27 2016, davem wrote:
As has been pointed out elsewhere in this ticket, an empty pattern
is interpreted as the "last successful match". The re-eval
mechanism takes this as being the currently executing pattern,
and so you get infinite recursion.

Marking the currently executing pattern as 'last successful'
is necessary within an re-eval so that things like $1 are visible:

#prints "a"
"ab" =~ /(.)(?{ print "[$1]\n" })/;