The number one program is Hello World.
This program prints the string "Hello World".
Strictly this is not part of the perl program - but it tells the
operating system what to do with the file. /usr/bin/perl is typically
where the perl executable is on a Linux system. It might be
c:/perl/bin/perl.exe on a Windows system.
This pragma makes perl check that variables have been declared,
you don't have to do this - but you should.