#!/usr/bin/perl
use strict;
use warnings;

my $str = do {local $/; <DATA>};
$str =~ s/\s+//g;

while ( $str =~ m/( (..+) .? (??{ reverse $2 }) )/xgc )
{
   print pos( $str ) . ": $1\n";
   pos( $str ) = $-[0] + 1;      # slide pos back to the left
}