#!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: