C:/Perl_524/bin\perl.exe  d:/tmp/pm/grep_autovivify.pl 
Use of uninitialized value $_ in warn at d:/tmp/pm/grep_autovivify.pl line 13.
x:  at d:/tmp/pm/grep_autovivify.pl line 13.
$VAR1 = [
          'c',
          'x',
          'b',
          'a'
        ];
Use of uninitialized value $_[0] in warn at d:/tmp/pm/grep_autovivify.pl line 24.
y:  at d:/tmp/pm/grep_autovivify.pl line 24.
$VAR1 = [
          'c',
          'x',
          'b',
          'a'
        ];