It does not need to be secure,

If it really doesn't need to be secure, merely a little obscure to discourage the curious and minimally motivated, you can get a quick and easy result in the same number of charactes, using just a little scrambling. Transform each character to a different character using tr///, and scramble the characters using an array slice. It should be fairly fast as well.

Update: Here's a very simple code implementation if that's helpful.

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";
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2nd Update: Xor with a pass phrase as syphilis uses can be used instead of the tr/// step in the code above, it's another way of doing and expressing the translation. Use of tr/// has an advantage if you want the $output confined to printable characters, otherwise, just decide which is easier for you to read.