Like Corion said, the actual error will be logged in the web server's error log. As far as the file location of Hello.pm, if it's simply in the same directory in the script, it's most likely that the file can't be found.

When you run the script on the command line from within the directory the script and module files are, everything will work. However, the web server executes the script from a different directory than that, which would result in Hello.pm not being found.

Either put the module file into a path that's in @INC, or do what was suggested and use lib ...;