Perl has a number of command line options, which you can specify on the command line by typing perl options hello.pl or which you can include in the shebang line. Let's say you want to use the -w command line option to turn on warnings:
#!/usr/bin/perl -w |
(Incidentally, it's always a good idea to turn on warnings while you're developing something.)
Setting the special variable $^W to a true value will locally disable warnings (i.e. in the current block).
A full explanation of command line options can be found in the Camel book on pages 330 to 337, or by typing perldoc perlrun.