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  1. or download this
    $ perl -MData::Dump -e '"ya"=~/(x)?/; dd $#-, \@-, $#+, \@+'
(0, [0], 1, [0, undef])
  2. or download this
    "1" =~ /1/    or die;
print $#-?"yes\n":"no\n";  # -> no
...
print $#-?"yes\n":"no\n";  # -> no !!
"1" =~ /(2)?/ or die;
print $#+?"yes\n":"no\n";  # -> yes