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use strict;
use warnings;
...
my @pairs = $s =~ /(?=(\w+ \s+ \w+))\w+ \s+/gx;

print join "\n", @pairs;

This is
is a
a test

...
my @pairs = map [split], $s =~ /(?=(\w+ \s+ \w+))\w+ \s+/gx;

print "@$_\n" for @pairs;

This is
is a
a test