sub foo {
    my ($s1, $s2) = @_;

    my @s1 = split //, $s1;
    my @s2 = split //, $s2;

    foreach my $idx ( 0 .. $#s1 ) {
        if ( $s1[$idx] eq chr(0) ) {
            $s1[$idx] = $s2[$idx];
        }
    }

    return join '', @s1;
}