Welcome!

The Perl CGI script is being run under the userid of the webserver (I'm assuming this is Apache under FreeBSD or Linux?), not your own. The directory where your file is to be created/opened needs to be writable by the webserver.

First, find out what userid the httpd daemons are running as: 
$ ps -aux | grep httpd
root   17714  0.0  0.2  5184  4456  ??  Ss    7Jun06   0:12.49 /usr/lo
+cal/sbin/httpd
www    17715  0.0  0.4  9164  8272  ??  I     7Jun06   0:00.24 /usr/lo
+cal/sbin/httpd
www    17716  0.0  0.4  8744  7864  ??  I     7Jun06   0:00.15 /usr/lo
+cal/sbin/httpd
www    17717  0.0  0.4  8768  7996  ??  I     7Jun06   0:00.19 /usr/lo
+cal/sbin/httpd
www    17724  0.0  0.4  8748  7788  ??  I     7Jun06   0:00.19 /usr/lo
+cal/sbin/httpd
www    17756  0.0  0.4  8772  7820  ??  I     7Jun06   0:00.20 /usr/lo
+cal/sbin/httpd
dwilde  7928  0.0  0.0  1512   988  p7  S+    6:28AM   0:00.00 grep ht
+tpd

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You'll note that the primary (on my system) is running as root, the rest as user 'www'. Therefore, the directory on the filesystem -- and it should be fully specified, such as '/var/web/root/logs/mylog.log' -- needs to be writable by www, or whatever your server is set up as. Apache configurations are very flexible, and it's equally likely your server will be running under user 'nobody'. In any event, make writing possible: 
$ su
Password:
# chown www /var/web/root/logs
# chmod 744 /var/web/root/logs
# exit
$

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As has been mentioned, allowing somebody to open a file on your server and write to it can cause mucho heartburn. Be very careful what you allow and who you allow to do it. I'd suggest that you force the directory and only allow the filename to be chosen: 
 my $subject = param('subject');
 open (LOG, ">/var/web/root/logs/$subject") || die "Error opening file
+: $!\n";

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And make sure that your script is limited as to how much information can be written to the file. Bear in mind that Perl can be used to poke a webserver as well as being poked, and some nasty little script kiddie will surely use it to do so at some point in your life. =8^O

Don Wilde
"There's more than one level to any answer."

In reply to Re: use variable as name for new file by samizdat
in thread use variable as name for new file by Zcity

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