Why use a hash when tr does the job so simply and efficiently?:

sub rot13{ 
    my $text = shift; 
    $text =~ tr[a-zA-Z][n-za-mN-ZA-M]; 
    return $text 
};;

$msg = 'The quick brown fox jumps over the lazy dog. And then runs awa
+y.';;

print rot13( $msg );;
Gur dhvpx oebja sbk whzcf bire gur ynml qbt. Naq gura ehaf njnl.

print rot13( rot13( $msg ) );;
The quick brown fox jumps over the lazy dog. And then runs away.
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