if ( my (@caps) = ($line =~ $re) ) {
    @caps = () if $caps[0] ne $1;           # reset pseudo captures
    $cb->(@caps);
    last;
}

##</code><code>##

c:\@Work\Perl\monks>perl

use strict;
use warnings;

use Data::Dump qw(pp);

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

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

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

        print "str <$str>  re $re  ";

        if ( my (@caps) = ($str =~ $re) ) {
          # @caps = () if $caps[0] ne $1;  # reset pseudo capture
            $#caps = $#- - 1;  # reset pseudo capture
            $cb->(@caps);
            }
        else {
            print 'NO match';
            }

        print "\n";

        }

    }

^Z
str <AB>  re (?-xism:..)  matched,  captured ()
str <AB>  re (?-xism:(.)(.))  matched,  captured ("A", "B")
str <AB>  re (?-xism:(X)?(.))  matched,  captured (undef, "A")
str <AB>  re (?-xism:XY)  NO match
str <AB>  re (?-xism:(X)Y)  NO match
str <AB>  re (?-xism:)  matched,  captured ()
str <>  re (?-xism:..)  NO match
str <>  re (?-xism:(.)(.))  NO match
str <>  re (?-xism:(X)?(.))  NO match
str <>  re (?-xism:XY)  NO match
str <>  re (?-xism:(X)Y)  NO match
str <>  re (?-xism:)  matched,  captured ()