#! perl -slw
use strict;

sub rndStr{local $"=''; "@_[map{rand$#_} 1 .. shift]"; }  

print rndStr 40, 'a'..'z';
print rndStr 40, 'A'..'Z';
print rndStr 40, 0..9;
print rndStr 40, 'a'..'z', 'A'..'Z', 0..9;

__END__
C:\test>212859
rxshjxkkgjqhyhukodujhmghpwujqteudxrvxsxo
MCRFGFETIDWEDHBWVSDKGJVMOEDEWKIJXRKAAGSQ
3288712881034147833223824725784363650780
EL0A5PsUCzPb8Sh3IyFGrCKElRPPeilXTPQDreKz

C:\test>