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# Return the list of dead cells surrounding a cell
sub get_dead_cells {
...
     ( $k8 = "$x2:$y2" ) x !( 0 + exists $cells->{ $k8 } ) );
}

use feature 'state';

...
     ( $k8 = "$x2:$y2" ) x !( 0 + exists $cells->{ $k8 } ) );
}