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my %mapping = (
  "a" => "1",  "b" =>  "2", "c" =>  "3", "d" >   "4", 
...
  "q" => "17", "r" => "18", "s" => "19", "t" => "20",
  "u" => "21", "v" => "22", "w" => "23", "x" => "24", 
  "y" => "25", "z" => "26"

my $new_ch = ( $old_ch - 'a' ) + 1;

my $new_ch = chr( ord( $old_ch ) - 96 );