How about a Schwartzian Transform? 
use strict;
use warnings;

my @array = qw(2aa 2ba 12kf 9cn 9vn 21sg);
my @sorted = 
   map  { join('',@$_) }
   sort { $b->[0] <=> $a->[0] or $a->[1] cmp $b->[1] } 
   map  { /(\d+)(\w+)/;[$1,$2] } @array
;
print "$_\n" foreach @sorted;

__END__
21sg
12kf
9cn
9vn
2aa
2ba

[download]
Update: be sure and read japhy's Resorting to Sorting. Especially the part about the Guttman Rosler transforms. (i'll leave that implementation of this particular problem as an exercise for another monk ;))

Update2: just noticed that the map after the sort could be written more consisely as: 

map  { [/(\d+)(\w+)/] } @array

[download]
Update3: fixed typo (s/b->\[1/a->[1/) ... thanks Enlil :)

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

In reply to (jeffa) Re: Helping with sorting by jeffa
in thread Helping with sorting by kiat

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