? So you want  @x = qw( first third ) ;

This "works"

dd( 'bogus firstblahjokeblahthird bogus' =~ m/
(first)
(?&patblah)
.*?
(?&patblah)
(third)
(?(DEFINE)
    (?<patblah>blah)
)
/sx );
__END__
("first", "third", undef)
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The undef comes from the named capture  (?<patblah>blah) which takes up slot $3 in this pattern, which is why (?(DEFINE) is placed at the end