This uses a look-ahead but in a slightly different way to blokhead's solution. The regex looks for and captures a letter sequence then uses a look-ahead with a capture to find another sequence.

use strict;
use warnings;

my $string = q{abc de fgh ijkl};

my $rxComb = qr
   {(?x)
       (?:
           ([a-z]+)
           \s+
           (?=([a-z]+))
       )
   };

while ( $string =~ m{$rxComb}g )
{
    print qq{$1 $2\n};
}
[download]

Here's the output.

abc de
de fgh
fgh ijkl
[download]

I hope this is of use.

Cheers,

JohnGG