use warnings;
use strict;

my $in = 'Simpletestxy';
my $tr_ed = $in;

$tr_ed =~ tr/A-Za-z1-9/1-9a-zA-Z/; 
#complicate and add chars to the tr/// as you wish
                                     #  0  1 2 3 4 5  6 7 8  9 10 11  
my $out  = join '',(split '',$tr_ed)[qw(5  6 4 2 9 10 3 7 11 1 8  0)];

my $back = join '',(split '',$out  )[qw(11 9 3 6 2 0  1 7 10 4 5  8)];
$back =~ tr/1-9a-zA-Z/A-Za-z1-9/; 

print "in=$in, tr_ed=$tr_ed, out=$out, back=$back\n";