How about this?

use warnings;
use strict;

my $string = "abc de fgh ijkl";
my @array = split /\s+/, $string;
my $count = 0;
while ($count < $#array) {
  print $"array[$count] $array[$count+1]\n";
  $count++;
}
[download]

It's not a regexp, but, it displays the output you want.

UPDATE: when you read the entire OP, you find funny things... like the fact that the OP-er listed this solution. Oh, well, made me not have to do real work.