That script uses the following perl flags:

-a autosplit mode with -n or -p (splits $_ into @F)
-n assume "while (<>) { ... }" loop around program
-i[extension] edit <> files in place (makes backup if extension supplied)

in addition to the standard "-e" for execute, and -M to include the module.

The executable code starts with a BEGIN{} block that sets up the current date/time into $my_dt.

The first statement uses the auto-split input line, and extracts the date pieces into @dt.

After that, it is a matter of comparing the dates, and deciding whether to print to stdout.

Your key to using it is to figure out how to correctly populate @dt, based on the contents of @F[0..2].