You should consider ditching the regex approach and using a module: Text::Ngrams looks like the right one for you (not to be confused with Text::Ngram -- note the "singular" -- which only works on character ngrams, not word ngrams).

If you have some strange compulsion not to use a module, you should still consider ditching the regex -- split the string into an array of words and use a for loop to output your word pairs:

my @words = split " ", $text;
print "$words[$_-1] $words[$_]\n"  for ( 1 .. $#words );
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(updated to fix bone-headed error in for loop)