if ( my (@caps) = ($line =~ $re) ) {
        no warnings 'uninitialized';
        @caps = () if $caps[0] ne $1;           # reset pseudo captures
        $cb->(@caps);
        last;
    }

##</code><code>##

    my (@matches) = ($line =~ $re)
    if (defined $&) {
        $cb->(@matches);
        last;
    }

##</code><code>##

use v5.12;
use warnings;

for my $str ("AB","") {
    say "****** str=<$str>";
    for my $re ( qr/../, qr/(.)(.)/, q/XY/, q/(X)Y/, q// ) {
        say "--- re=<$re>";
        my @captures = $str =~ $re;
        if ( defined $& ) {
            say "matched"
        } else {
            say "no match"
        }
        if (defined $1) {
            say "with captures <@captures>";
        } else {
            say "no captures";
        }
    }
}

##</code><code>##


****** str=<AB>
--- re=<(?^u:..)>
matched
no captures
--- re=<(?^u:(.)(.))>
matched
with captures <A B>
--- re=<XY>
no match
no captures
--- re=<(X)Y>
no match
no captures
--- re=<>
matched
no captures
****** str=<>
--- re=<(?^u:..)>
no match
no captures
--- re=<(?^u:(.)(.))>
no match
no captures
--- re=<XY>
no match
no captures
--- re=<(X)Y>
no match
no captures
--- re=<>
matched
no captures