@array_of_test_names = (
  ...
);;

$mask = '';;
$mask ^= $_ for @array_of_test_names;;
$mask ^= $array_of_test_names[0] if @array_to_test_names &1'

$mask =~ m[^(\0+)] and $len = length( $1 );;

print substr $array_of_test_names[ 0 ], 0, $len;;
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