There are 2 pages involved. The first is a form that passes the value $newvalue to the CGI script. The CGI script receives that value fine and I can dump it to screen fine.

If I try and write that value to the config file one of two things happens. Firstly, if I single quote the value '$newvalue' it will write the literal string $newvalue to the config file. So this rules out any permissions issues with writing to the file.

Any other format ("$newvalue" or \$newvalue) will not write to the file at all. It simply ignores it.

If I set the variable value myself in the CGI script (that is, not pass it from a form) and then try and write that value to the file it works.