in reply to where to place the .pl fro cgi script
The answers previously given are quite correct, of course, but the precise answer to your question is that hello.pl must be placed in the directory cgi-bin.
Typically this will be found in the public_html directory, but is sometimes not so you may have to look around for it
You also may need to change the permissions on the file to 755 (normally this can be done using you ftp client) and if you are viewing from a browser an html header print "Content-type: text/html\n\n"; before your print 'hello world'; statement will probably be necessary.
Should the script not function after all this come back to us and we'll try to be of more help.