You could go (almost) without regexes and just get the pairs yourself - at the cost of some memory (since you need to store all the separate words):

#!/usr/local/bin/perl -w
use strict;
my $s = 'This is a test';

my @words = split / +/,$s;

my @pairs = map [@words[$_,$_+1]],0 .. @words-2;
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