Exactly. Use bracketing construct like this:

if (/\b(API|APIN|UWI)\s*\./) { somesub($1); }
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Read perlre on saving parentheses.

Update: note that my regexp isn't equal to yours. It instead matches what you probably wanted it to match. The difference is due to the vertical bar | construct. Your original regexp matches \bAPI or APIN or UWI\s*\..