#!perl
use strict;

open INPUT,...
while(<INPUT>)
{
...code inside loop...
}

##</code><code>##

#!perl -n
use strict;
...code inside loop...

##</code><code>##

#!perl -n
use strict;
...code to do with every line of input stored in $_...
print $_; # or just print

##</code><code>##

#!perl -p
use strict;
...code to do with every line of input stored in $_...