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use strict;
use warnings;
...
    substr $str, $_, 1, q{x} for @$raToMask;
    return $str;
}

              Rate    andRegex andOrAndNot  substrList
andRegex    12.8/s          --        -43%        -50%
andOrAndNot 22.6/s         77%          --        -11%
substrList  25.4/s         99%         12%          --