ikegami,

I tried the lexical order method, and it's almost what I need...

On a mac, I get this output:

FMMNllltuwwxxxz
13AEITfhlllstxx
/39CMMNhhlllllluuwwxxx
8ELMMfhllllxxx
49MMdhllllouxx
2239MMlllquxxxx
018AMMPPQTbbffhllllx
/2559Mllrtuuwwxz
[download]

If I change $seen{$_}++ to $seen{$_}+=2 then I get this:

/034589BDFFHKMMMNNUXabcfghjllllnqsttuuwwwxxxxyzz
/112334589AACEEGIIKLMNPQRSTTbdeffhhjkllllnopqssttuxxx
//01335899ABCCIMMMNNRTVYbcdefghhhjkllllllloqtuuuwwwxxxxy
/035889BEEFKLLMMMNOPQTabcffghhlllllmqrtuvwxxxx
+/134456899ACGKMMMNOQWbddghhlllllmnooqrtuuwxxx
/2223345899ABDEFGHIMMMNORTabdfhillllnqqtuuwxxxxxyz
+/00113457889AACEIMMMNPPPQQTTZabbbcefffghhlllllmqtuwxx
//2234555899ACGILMMNOQUbehlllmoqrrsttuuuwwwxxyzz
[download]

which DOES list all the chars used, but has duplicates.