> $1 will be undefined if there is no capture group 1

I was aware of that and my results are exactly like yours.

In the following I just need to silence the warnings about undef values.

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";

    }

}
[download]

--- str <AB>  re (?^u:..)  
matched,  captured ()
matched,  captured ()


--- str <AB>  re (?^u:(.)(.))  
matched,  captured ("A", "B")
matched,  captured ("A", "B")


--- str <AB>  re (?^u:(X)?(.))  
matched,  captured (undef, "A")
matched,  captured (undef, "A")


--- str <AB>  re (?^u:XY)  
NO match


--- str <AB>  re (?^u:(X)Y)  
NO match


--- str <AB>  re (?^u:)  
matched,  captured ()
matched,  captured ()


--- str <>  re (?^u:..)  
NO match


--- str <>  re (?^u:(.)(.))  
NO match


--- str <>  re (?^u:(X)?(.))  
NO match


--- str <>  re (?^u:XY)  
NO match


--- str <>  re (?^u:(X)Y)  
NO match


--- str <>  re (?^u:)  
matched,  captured ()
matched,  captured ()
[download]

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