blazar: 

Now, is this test flawed?

You are basically correct here. I was too zealous here to advertise the vantages of index() and tr//. They have their run elsewhere, but not in this special case. Thanks for pointing this out.

I abused your benchmark code (of course) to find out on how good the index() optimization in Perl5 really is ;-)

...
use Benchmark qw/cmpthese :hireswallclock/;

my @a = map { my $s='PM is cool, ' x 10_000;
              substr($s, rand(length $s), 1, '_');
              $s
            } 1..1000;

cmpthese -3 => {
                C_Idx => sub () { grep C_Idx($_, '_') < 0, @a },
                Index => sub () { grep index($_, '_') < 0, @a },
                Regex => sub () { grep ! /_/,    @a },
                Tr    => sub () { grep ! tr/_//, @a }
               };

use Inline C => qq[
 int C_Idx(SV* src, SV* chr)
{
 STRLEN srclen, chrlen;
 char *ssrc = SvPV(src, srclen), *schr = SvPV(chr, chrlen);
 char *p = ssrc;
 if( chrlen != 1 ) croak("single characters only for now!");
 return (p=memchr(p, *schr, srclen)) != NULL ? p-ssrc : -1;
}
];
...
[download]

On my system, somehow above 60-70K strings - the index() falls behind the c-library function for finding a character (memchr). For the above strings:

        Rate    Tr Regex Index C_Idx
Tr    3.17/s    --  -74%  -74%  -87%
Regex 12.2/s  284%    --   -0%  -52%
Index 12.2/s  285%    0%    --  -52%
C_Idx 25.2/s  696%  107%  107%    --
[download]

I personally believe it'd be much better If I'd read my own posts and think about their assumptions next time much more thoroughly ;-)

Regards

mwa