I got the sense that you didn't want just the captured parts. But if you only need the $1 from each match,

map { /^..(A|B)./ ? [ $_, $1 ] : () } @a
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simplifies to

map { /^..(A|B)./ ? $1 : () } @a
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but you could also go a step further and simply use

map /^..(A|B)./, @a
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(I added this to the original post to show the options in one place.)