my $change_next;
while (<>) {
    if ( /<binddn>cn=xyzzz</ ) {
        $change_next = 1;
    }
    elsif ( $change_next and /<bindpass>welcome1</ ) {
        s/1/123/;
        $change_next = undef;
    }
    print;
}

##</code><code>##

    perl name_of_script < old_file.xml > new_file.xml