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    if ( my (@caps) = ($line =~ $re) ) {
        no warnings 'uninitialized';
...
        $cb->(@caps);
        last;
    }

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

use v5.12;
use warnings;
...
        }
    }
}

****** str=<AB>
...
matched
no captures