c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'ABCDEF';
 ;;
 my @triplets;
 $s =~ s{ (?= (...)) . }{ push @triplets, $1;  ''; }xmsge;
 ;;
 print qq{'$s'};
 printf qq{'$_'  } for @triplets;
"
'EF'
'ABC'  'BCD'  'CDE'  'DEF'

##</code><code>##

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'ABCDEF';
 ;;
 local our @triplets;
 $s =~ s{ (?= (...) (?{ push @triplets, $^N })) . }''xmsg;
 ;;
 print qq{'$s'};
 printf qq{'$_'  } for @triplets;
"
'EF'
'ABC'  'BCD'  'CDE'  'DEF'