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#!/usr/bin/env perl
use warnings;
...
  $sub{$name} = $code;
}

# The original subroutine

...
  join '', @rotd{split('',shift)};
};

# Two variants instead of the split

...
  join '', @rotd{unpack "(a)*", $x};
};

# Now let's use an array slice instead of a hash

...
  $s;
};

# First, check correctness.  This is important.

...
in, dictum in, blandit eu, sapien. Morbi vestibulum, metus eu auctor v
+ulputate,
nulla lectus condimentum nisi, ac pulvinar ligula nunc in felis. Curab
+itur id
orci ac est luctus molestie.

                Rate   orig hashslice-m gsub-dot hashslice-s gsub-clas
+s hashslice-u arrayslice   tr
orig           962/s     --        -20%     -30%        -40%       -43
+%        -58%       -82% -99%
...
hashslice-u   2278/s   137%         89%      66%         42%        35
+%          --       -58% -98%
arrayslice    5414/s   463%        349%     295%        237%       221
+%        138%         -- -96%
tr          148174/s 15308%      12184%   10701%       9135%      8681
+%       6403%      2637%   --