In addition to what others said, if you really needed

my %mapping = (
  "a" => "1",  "b" =>  "2", "c" =>  "3", "d" >   "4", 
  "e" => "5",  "f" =>  "6", "g" =>  "7", "h" =>  "8", 
  "i" => "9",  "j" => "10", "k" => "11", "l" => "12", 
  "m" => "13", "n" => "14", "0" => "15", "p" => "16", 
  "q" => "17", "r" => "18", "s" => "19", "t" => "20",
  "u" => "21", "v" => "22", "w" => "23", "x" => "24", 
  "y" => "25", "z" => "26"
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you could replace it with (untested)

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

my $new_ch = chr( ord( $old_ch ) - 96 );
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Perl golfers, and those more confident than me, can reduce itr further.