I've tried. do 'admin.pl?msg=Success'

Try the script on the command line. If it is called the way you say, on the command line it will be 'admin.pl msg Success' or 'perl admin.pl msg Success' with spaces between the name(s) and argument(s).

If that works, try the same with backticks or   qx()   from the CGI script.
If that doesn't work, but the latter did work, you are having some sort of permission issue.

You could also think about using Location headers to call the other script the HTTP way by help of the browser.
Another way still would be to use LWP::Simple.

Cheers, Sören