vec( $vector, $bit, 1 ) ||= 1;

##</code><code>##

use strict;
use warnings;
use Benchmark qw[ cmpthese ];
use Inline C => 'DATA';

my( $vec1, $vec2 ) = ( ( chr(0) x 125000 ) x 2 );
cmpthese -3, {
    inline => sub { mytest( $vec1, $_ )       for 0 .. 1e6-1; },
    vec    => sub { vec( $vec2, $_, 1 ) ||= 1 for 0 .. 1e6-1; },
};

warn "Different results" unless $vec1 eq $vec2;

__DATA__
__C__

int mytest(SV* sv_vec, unsigned int bit) {

    STRLEN vecbytes; // Length of vector in bytes
    unsigned char *myvec = (unsigned char *) SvPV(sv_vec, vecbytes);

    if (bit/8 >= vecbytes) return 0; // Check in range

    if (myvec[bit/8] & 1U<<(bit%8)) return 1; // Test if a bit is set

    myvec[bit/8] |= 1U<<(bit%8); // Set bit (CHANGES $vector)

    return 1;
}

##</code><code>##

C:\test>903727.pl
         Rate inline    vec
inline 3.11/s     --   -60%
vec    7.77/s   150%     --

C:\test>903727.pl
         Rate inline    vec
inline 3.10/s     --   -60%
vec    7.77/s   151%     --