use Inline C => qq{
SV* swap(SV* one, SV* two) {
    char *buf1, *buf2, *buf3;
    STRLEN idx, len;
    SV* ret;

    if (!SvPOK(one) || !SvPOK(two) || sv_len(one) > sv_len(two))
        return newSVsv(&PL_sv_undef);

    len = sv_len(one);
    buf1 = SvPVX(one);
    buf2 = SvPVX(two);
    buf3 = malloc(len);

    for (idx=0; idx < len; idx++) {
        buf3[idx] = buf1[idx] ? buf1[idx] : buf2[idx];
    }

    ret = newSVpv(buf3, len);
    free(buf3);
    return ret;
}

};

# [ your code, but with ]
cmpthese( -5, {
    'split1' => sub { my $s3 = split1( $s1, $s2 ) },
    'substr1' => sub { my $s3 = substr1( $s1, $s2 ) },
    'inline' => sub { my $s3 = swap($s1, $s2) },
});