It is one of the things I consider to be a bug in Perl's regex engine. Zero-width matches can cause Perl to consider multiple matches starting at the same point. Perl eventually rejects identical matches in order to prevent an infinite loop. Your (?{ block catches Perl given a go at trying to match something different by trying the match a second time at each starting point.

You can work around this simply enough:

#!perl -wl
 use strict;
 my $s = 'abcd';

 my @pairs1 = $s =~ m{ (?= (..)).? (?{ printf qq{ '$^N'} }) }xmsg;
#                               ^^
 print '';
 printf qq{ :$_:} for @pairs1;
 print '';

 my @pairs2 = $s =~ m{ (?= (..) (?{ printf qq{ '$^N'} })).? }xmsg;
#                                                        ^^
 print '';
 printf qq{ :$_:} for @pairs2;
 print '';
__END__
 'ab' 'bc' 'cd'
 :ab: :bc: :cd:
 'ab' 'bc' 'cd'
 :ab: :bc: :cd:
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(Update: Pasted the wrong code for a few seconds.)