Take a look at the documentation for the -p and -n switches in perlrun. In prasee er...short, it says

-p
causes Perl to assume the following loop around your program, which ma
+kes it iterate over filename arguments somewhat like sed:

LINE:while (<>) { # your program goes here } 
continue { print or die "-p destination: $!\n"; }
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If you substitute the 'program' eg. "}{die" where the comment above says " your program goes here", then you willl see how the thing works.