We keep notes on some of the useful notes on perl command line knowledge here.

1.-i[extension]:

specifies that files processed by the “<>” construct are to be edited in-place. It does this by renaming the input file, opening the output file by the original name, and selecting that output file as the default for print() statements.

2.-p: causes Perl to assume the following loop around your program, which makes it iterate over filename arguments somewhat like sed.