... if the regex fails on the first value, then the scalar is undef ...

I agree that if you want processing to stop, already, after the first match, a for-loop is the way to go.

However, in the code you're using in your example map blocks, the value assigned to a scalar in list-context in the event of match failure would be the empty string, not undef.

(I should mention that even though the OP is a bit unclear on this point, I assume that the  @GET_STRING array may contain any number of matching or non-matching strings in any order.)