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};
}

##</code><code>##

abc de
de fgh
fgh ijkl