Benchmark: timing 100000 iterations of csimple3, simple3...
  csimple3:  5 wallclock secs ( 4.43 usr +  0.00 sys =  4.43 CPU) 
                   @ 22573.36/s (n=100000)
   simple3: 19 wallclock secs (16.90 usr +  0.01 sys = 16.91 CPU) 
                   @ 5913.66/s (n=100000)

##</code><code>##

#!/usr/bin/perl

use Benchmark;
use Inline C => 'DATA',
  VERSION => 0.0,
  NAME => 'SimpleTest',
  OPTIMIZE => '-O3';
Inline->init;

sub simple3
{
  my($a,$b)=@_;
  my(@seen);

  return undef if (length($a) != length($b));
  foreach my $i (0..length($a))
  {
    my($ac,$bc)=(substr($a,$i,1),substr($b,$i,1));
    if ($ac eq $bc)
    {
      ; # Do nothing
    }
    elsif ($ac eq '_')
    {
      return undef if (++$seen[$bc] > 1);
    }
    elsif ($bc eq '_')
    {
      return undef if (++$seen[$ac] > 1);
    }
    else { return undef }
  }
  1;
}

my @tests =
(qw/
     _8__3__19
     48____7__

     _8__3__19
     4_2___7__

     _8__3__19
     4_8___7__

     __8_3__19
     48____7__

     __8_3__19
     84____7__

     _8__3__19
     48_____7_
   /
);

sub run_tests
{
  my($func,$verbose,@tests)=@_;
  my ($s1, $s2);
  while (defined($s1 = shift(@tests)))
  {
      $s2 = shift(@tests);
      my $result = $func->($s1, $s2);
      print "$s1\n$s2: ",$result?"compatible":"not compatible","\n"
        if ($verbose);
  }
}

run_tests(\&csimple3, 1, @tests);

timethese(100_000, {
                 simple3 => sub { run_tests(\&simple3, 0, @tests) },
                 csimple3 => sub { run_tests(\&csimple3, 0, @tests) },
                });

__DATA__
__C__
int csimple3(const char *a, const char *b)
{
  int i;
  int l;
  unsigned char seen[256];

  memset(seen,0,256);

  if ((l=strlen(a)) != strlen(b))
    return 0;
  for(i=0;i<l;i++)
  {
    if (a[i] == b[i])
    {
      ; /* Do nothing */
    }
    else if (a[i] == '_')
    {
      if (++seen[b[i]] > 1)
        return 0;
    }
    else if (b[i] == '_')
    {
      if (++seen[a[i]] > 1)
        return 0;
    }
    else
      return 0;
  }
  return 1;
}