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```
    $v = $_ for 1, 2, 3;
```

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    $ perl -wle 'm[^(?{undef $var})(?:([01])(?{$var //= $^N}))+$] 
      and print "$_: '"'"'$var'"'"'" for qw[ 0 1 00 11 10 01 012]'
...
    11: '1'
    10: '1'
    01: '0'