I came up with a similar I::C implementation. I'd run it against yours but yours doesn't compile on my machine at the moment:

int indexNth( SV *haystack, SV*needle, int nth ) {
    STRLEN lh, ln, i, j;
    char *h = SvPV( haystack, lh );
    char *n = SvPV( needle, ln );

    for( i = 0; i < lh; ++i ) {
        for( j = 0; j < ln; ++j ) {
            if( h[ i + j ] != n[ j ] ) goto nomatch;
        }
        if( --nth == 0 ) return i;
        nomatch:;
    }
    return -1;
}
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Got the C++ to compile -- missing newline at the end of the file, These are the results:

C:\test>1039228.pl
        Rate  regex    buk    cpp      c
regex 4.06/s     --   -97%   -99%  -100%
buk    137/s  3277%     --   -75%   -85%
cpp    546/s 13345%   298%     --   -40%
c      915/s 22446%   568%    68%     --
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And the additional test:

sub c {
  my $copy = $main::string;
  my $ix = indexNth( $copy, '!', 50000 );
  substr( $copy, 0, $ix ) =~ tr/!/\n/;
  return \$copy;
}
[download]