sub mrm_4 {
    # from [bart]'s vec()
    my ($s1, $s2) = @_;
    use bytes;

    my $pos = 0;
    while ( -1 < ( $pos = index $$s1, '\0', $pos ) ) {
        vec( $$s1, $pos, 8 ) ||= vec( $s2, $pos, 8 );
    }
}

sub mrm_5 {
    # from moritz's, seeing if four-arg substr() is
    # faster or slower than lvalue substr()
    my ( $s1, $s2 ) = @_;
    use bytes;
    my $pos = 0;
    while ( -1 < ( $pos = index $$s1, '\0', $pos ) ) {
        substr( $$s1, $pos, 1, substr( $s2, $pos, 1 ) );
    }
}