And of course, the other approach (if you don't have 5.10+) is to just grep away all undefined values:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $s = 'foo [21a] plus (b23) baz';
 ;;
 my @captures =
   grep defined,
   $s =~ m{ (\d+) (a|b) | (a|b) (\d+) }xmsg
   ;
 dd \@captures;
"
[21, "a", "b", 23]
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