use v5.12;
use warnings;

use Data::Dump qw(pp);


my $cb = sub { say "matched,  captured ", pp @_; };

for my $str ("AB","") {

    for my $re (qr/../, qr/(.)(.)/, qr/(X)?(.)/, qr/XY/, qr/(X)Y/, qr//) {

        say "--- str <$str>  re $re  ";

        if ( my (@caps_0) = ($str =~ $re) ) {

            my @caps = @caps_0;
            $#caps = $#- - 1;           # reset pseudo capture
            $cb->(@caps);

            @caps = @caps_0;
            no warnings 'uninitialized';
            @caps = () if $caps[0] ne $1; # reset pseudo capture
            $cb->(@caps);

        } else {
            say 'NO match';
        }

        say "\n";

    }

}