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my %rot;
@rot{'A'..'M'} = ('N'..'Z');
...
   s/([$chars_to_change])/$rot{$1}/g;
   return $_;
}

my %rot;
@rot{'A'..'M'} = ('N'..'Z');
...
     {' . join('', map quotemeta, values %rot) . '};
   return $_;
}';

Benchmark: running rot13builtre, rot13builttr, rot13dynre, rot13dynre2
+, rot13splitmap, rot13substrfor, rot13tr, each for at least 3 CPU sec
+onds...
rot13builtre:    3 wallclock secs ( 3.13 usr +  0.00 sys =  3.13 CPU) 
+@   572.75/s (n=  1795)
...
rot13builtre     573/s           57%            17%          2%       
+  1%           --    -99%         -99%
rot13tr        61032/s        16622%         12326%      10746%     10
+649%       10556%      --          -1%
rot13builttr   61540/s        16760%         12429%      10836%     10
+738%       10645%      1%           --

#!/usr/bin/perl
use strict;
...
Morbi vestibulum, metus eu auctor vulputate, nulla lectus condimentum
nisi, ac pulvinar ligula nunc in felis. Curabitur id orci ac est
luctus molestie.