use strict;
use warnings;
use Data::Dump 'dd';

my $s = "a aa aaa aaaa";

$s =~ /(?<a>a+) (?<a>a+) (?:(?<a>a+)bbb)?/;

dd \@{^CAPTURE};        # all captured groups
dd $+{a};               # leftmost defined "a"
dd ${^CAPTURE}{a};      # ditto
dd $-{a};               # all defined "a"'s groups
dd ${^CAPTURE_ALL}{a};  # ditto

__END__

["a", "aa"]             # correct
"a"                     # correct
undef
["a", "aa", undef]
"a"