The optomiser eliminates any differences. Yours (c_luhn3 below) can be slightly faster one run and slightly slower on the next; but the difference is less that 0.5% either way:

#! perl -slw
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => '_luhn', CLEAN_AFTER_BUILD =>0;

int c_luhn( char *s ) {
    int i, total = 0;

    for( i=0; i < 15; ++i ) {
        int d = s[ i ] - '0';
        if( !( i  & 1 ) ) {
            d *= 2;
            if( d > 9 ) d -= 9;
        }
        total += d;
    }
    total *= 9;
    return total % 10;
}

int l[] = { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };
int c_luhn2( char *s ) {
    int total = 0;

    total += l[ s[ 0] - '0' ];
    total +=    s[ 1] - '0';
    total += l[ s[ 2] - '0' ];
    total +=    s[ 3] - '0';
    total += l[ s[ 4] - '0' ];
    total +=    s[ 5] - '0';
    total += l[ s[ 6] - '0' ];
    total +=    s[ 7] - '0';
    total += l[ s[ 8] - '0' ];
    total +=    s[ 9] - '0';
    total += l[ s[10] - '0' ];
    total +=    s[11] - '0';
    total += l[ s[12] - '0' ];
    total +=    s[13] - '0';
    total += l[ s[14] - '0' ];

    total *= 9;
    return total % 10;
}

int ll[] = {
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };

int
c_luhn3( char *s ) {
    return ((
        ll[ (int)s[ 0] ]
      +    s[ 1]
      + ll[ (int)s[ 2] ]
      +    s[ 3]
      + ll[ (int)s[ 4] ]
      +    s[ 5]
      + ll[ (int)s[ 6] ]
      +    s[ 7]
      + ll[ (int)s[ 8] ]
      +    s[ 9]
      + ll[ (int)s[10] ]
      +    s[11]
      + ll[ (int)s[12] ]
      +    s[13]
      + ll[ (int)s[14] ] - 7 * '0' ) * 9) % 10;
}

END_C
use Time::HiRes qw[ time ];

my @samples = qw[
    4011350000000008
    4011350000000016
    4011350000000024
    4011350000000032
    4011350000000040
    4011350000000057
    4011350000000065
    4011350000000073
    4011350000000081
    4011350000000099
];

sub luhn {
    use integer;
    my $s = $_[ 0 ];
    my $total = 0;
    for my $i ( 0 .. 14 ) {
        my $d = substr( $s, $i, 1 );
        unless( $i & 1 ) {
            $d *= 2;
            $d -= 9 if $d > 9;
        }
        $total += $d;
    }
    $total *= 9;
    return chop $total;
}

for ( @samples ) {
    print "$_: ", luhn( substr $_, 0, 15 );
}

my $start = time;
#for ( 401135000000000..401135000999999 ) {
#    my $chk = luhn( $_ );
#}
#printf "Took %.9f seconds.\n", time() - $start;

#for ( @samples ) {
#    print "$_: ", c_luhn( $_ );
#}

$start = time;
for ( 401135000000000..401135000999999 ) {
    my $chk = c_luhn( $_ );
}
printf "Took %.9f seconds.\n", time() - $start;

for ( @samples ) {
    print "$_: ", c_luhn2( $_ );
}

$start = time;
for ( 401135000000000..401135000999999 ) {
    my $chk = c_luhn2( $_ );
}
printf "Took %.9f seconds.\n", time() - $start;

for ( @samples ) {
    print "$_: ", c_luhn3( $_ );
}

$start = time;
for ( 401135000000000..401135000999999 ) {
    my $chk = c_luhn3( $_ );
}
printf "Took %.9f seconds.\n", time() - $start;
[download]

C:\test>luhn
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.751899958 seconds.
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.695394039 seconds.
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.693738222 seconds.


C:\test>luhn
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.751657963 seconds.
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.694734097 seconds.
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.697062969 seconds.
[download]