First check the HTTP server's access and error logs. There will probably be a message in the error log saying that test.pl was not found, but you may find a more specific reason. If not, you will have to figure out why the server can't find (or can't open) the script.
There are several possibilities. First, check that the URL you use really corresponds to the directory where the script is. Commonly, a test.pl script would be in a cgi-bin directory and the URL needs to end in .../cgi-bin/test.pl.
Another possibility is that test.pl is not executable by the HTTP server; an ls -l will show you if the appropriate execute bits are missing, or if the file ownership isn't what you expected.