This is how I would do it, but I generally use a non-capturing group whenever possible for clarity:

if ( $a =~ /^ (?: ABA | SCO | ACC | PHC | GHF ) $/x ) { ... }
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Note that this is more for capturing intent than performance. When I see a capture group (...), I assume we will be using whatever was captured. In this case, since we are capturing the entire string, $a == $1.

Probably not worth the nitpick :)

Best,

Jim