FYI, You can also use the Perl Expect module to drive shell commands if just using backticks or system calls is not working out for you. But this whole thing is a bad idea, especially given your your level of coding experience.

Not only are you shelling out far, far too much but you have at least one huge security hole: 'echo "$passwordnew"'. Imagine if someone entered `/bin/rm -rf /` for the password!! Use single ticks in the echo command instead. (Update: added the bold italics)

But really, I think you should bail on this. It's a bad idea. Maybe tell us why it has to be a web page --there might be another way to do this.