Not using regex matches at all might be a better solution.

$_ = 'This is a test';
my @words = split;
my @wordpairs = map {[ $words[$_-1], $words[$_] ]} 1..$#words;
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Benchmarking is left as an exercise... ;)

Update: oops! I failed to notice that Joost already provided this solution. (I should have expected that he would.) Apologies for being redundant.