sub mrm_1 {
    my ( $s1, $s2 ) = @_;
    use bytes;
    my $pos = 0;
    while (-1 < ( $pos = index $$s1, '\0', $pos ) ) {
        substr( $$s1, $pos, 1 ) = substr( $s2, $pos, 1 );
    }
}

sub mrm_3 {
    my ( $s1, $s2 ) = @_;
    use bytes;

    my @zeros = ();
    my $pos = 0;
    while ( -1 < ( $pos = index $$s1, '\0', $pos ) ) {
        push @zeros, $pos;
    }
    for ( @zeros ) {
        substr( $$s1, $_, 1 ) = substr( $s2, $_, 1 );
    }
}