Nice snippet. I couldn't help but point out that you already have a built-in counter -- the number of elements in the array. Reworking your example a bit, we get the following:

use strict;                

my $randstring;
my @testarray = ( 'a' .. 'z', 'A' .. 'Z', '0' .. '999' );

$randstring .= splice(@testarray, rand @testarray, 1)
  while @testarray;

print "$randstring\n";
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Cheers,
Matt