Yes, you need to use parentheses, like this:

if (/\b(API|APIN|UWI)\s*\./)
{
    somesub($1);
}
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The parentheses group parts of the regular expression, in addition to "capturing" parts of it that you can use in subsequent code. Both functions are important here: I'm guessing that you actually wanted to match just the letters, not the surrounding whitespace. In other words, I'm guessing that you didn't want what your original expression was doing: matching one of

• \bAPI
• APIN
• UWI\s*\.

Because the alternation character | doesn't bind very tightly.

More information at perlre.

HTH