Just looking at the faster versions and removing everything that is not the function (ie one true, one false test makes a set)...

#!/usr/bin/perl

use Inline C =>;
use Benchmark 'cmpthese';

#               Rate  tchn tchn2  kval    LR  tye2  tye1
#    tchn   556060/s    --   -9%  -19%  -20%  -50%  -64%
#    tchn2  611783/s   10%    --  -11%  -12%  -45%  -60%
#    kval   690724/s   24%   13%    --   -0%  -38%  -55%
#    LR     691904/s   24%   13%    0%    --  -38%  -55%
#    tye2  1114446/s  100%   82%   61%   61%    --  -28%
#    tye1  1542899/s  177%  152%  123%  123%   38%    --

my $str_a = 'a'x255;
my $str_b = 'b'x255;
my $str_c = $str_a;
my ( $fc );


cmpthese -5, {
    'tchn' => sub {
        $a = same_scan( $str_a, $str_b );
        $a = same_scan( $str_a, $str_c );
    },
    'tchn2' => sub {
        $a = same_scan2( $str_a, $str_b );
        $a = same_scan2( $str_a, $str_c );
    },
    'LR' => sub {
        $a = index($str_a, substr($str_b, 0, 1));
        $a = index($str_a, substr($str_b, 0, 1));         
    },

    'tye1' => sub {
        $a = ord $str_a != ord $str_b;
        $a = ord $str_a != ord $str_c;
    },
    'tye2' => sub {
       $fc = ord $str_a;
       $a = $fc != ord( $str_b );
       $fc = ord $str_a;
       $a = $fc != ord( $str_b );
    },
    'kval' => sub {
        $fc = substr $str_a, 0, 1;
        $a = $fc ne substr $str_b, 0, 1;
        $fc = substr $str_a, 0, 1;
        $a = $fc ne substr $str_c, 0, 1;
    },
};
__END__
__C__
int same_scan(char* str1, char* str2)
{
  return str1[0] == str2[0] ? 1 : 0;
}

int same_scan2(SV* str1, SV* str2)
{
  return SvPV(str1, PL_na)[0] == SvPV(str2,PL_na)[0] ? 1 : 0;
}
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