# predefine $testlist as a simple string of the set of acceptable characters then
  'index' => sub {
                   $test_char = chr( int( rand 256 ) );
                   if (index($testlist, $test_char) > -1 && length($test_char)==1) {} else {}
                 },

##</code><code>##

Benchmark: timing 10000000 iterations of comp, exist, index, index2, regex, regex2, regex3...
      comp: 44 wallclock secs (41.64 usr +  0.00 sys = 41.64 CPU) @ 240147.93/s (n=10000000)
     exist: 41 wallclock secs (38.30 usr +  0.00 sys = 38.30 CPU) @ 261117.06/s (n=10000000)
     index: 42 wallclock secs (41.30 usr +  0.00 sys = 41.30 CPU) @ 242148.34/s (n=10000000)
    index2: 40 wallclock secs (39.48 usr +  0.00 sys = 39.48 CPU) @ 253260.73/s (n=10000000)
     regex: 55 wallclock secs (53.28 usr +  0.00 sys = 53.28 CPU) @ 187684.17/s (n=10000000)
    regex2: 53 wallclock secs (52.34 usr +  0.01 sys = 52.36 CPU) @ 190989.13/s (n=10000000)
    regex3: 53 wallclock secs (50.97 usr +  0.00 sys = 50.97 CPU) @ 196197.69/s (n=10000000)