The problem is you're escaping the delimiters, so they're being seen as capturing parens as opposed to literal parens e.g

print qr(\s+execution\s+time\s+\(\s+millis\s+\)\s+\:\s+);

__output__

(?-xism:\s+execution\s+time\s+(\s+millis\s+)\s+:\s+)
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A simple solution would is use an alternate quoting delimiter like square brackets or simpler still don't use a regex at all e.g

print join ' ', map { chomp; (split)[-1] } grep /\s+\d+$/, <DATA>;
print " Done!\n"
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