>perl -wMstrict -le
"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 '';
"
'ab' 'ab' 'bc' 'bc' 'cd' 'cd'
:ab: :bc: :cd:
'ab' 'ab' 'bc' 'bc' 'cd' 'cd'
:ab: :bc: :cd: