Just to clarify, when used in an alternation in list context, a capture group always returns something if it matches or not. If it does not match, undef is returned. One way to filter out these undefs is with a grep.

>perl -wMstrict -MData::Dump -le
"my $s = 'AAA BBB CCC DDD AAA BBB';
 ;;
 my @captures = $s =~ m{ (AAA) | (BBB) | (XXX) | (YYY) }xmsg;
 dd \@captures
 ;;
 my @matches = grep defined, $s =~ m{ (AAA) | (BBB) | (XXX) | (YYY) }x
+msg;
 dd \@matches;
"
[
  "AAA",
  undef,
  undef,
  undef,
  undef,
  "BBB",
  undef,
  undef,
  "AAA",
  undef,
  undef,
  undef,
  undef,
  "BBB",
  undef,
  undef,
]

["AAA", "BBB", "AAA", "BBB"]
[download]

Update: Another way to capture only matches is with the "branch reset" extended pattern (see  "(?|pattern)" in Extended Patterns in perlre) available with Perl version 5.10+.

>perl -wMstrict -MData::Dump -le
"use 5.010;
 ;;
 my $s = 'AAA BBB CCC DDD AAA BBB';
 ;;
 my @matches = $s =~ m{ (?| (AAA) | (BBB) | (XXX) | (YYY) ) }xmsg;
 dd \@matches
"
["AAA", "BBB", "AAA", "BBB"]
[download]