This is built-in behavior in Perl. From perlrun:
The null filehandle <> is special: it can be used to emulate the behavior of sed and awk. Input from <> comes either from standard input, or from each file listed on the command line. Here’s how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is empty, $ARGV[0] is set to "-", which when opened gives you standard input. The @ARGV array is then processed as a list of filenames.

In reply to Re^3: How to write a program to read stdin or command line? by perrin
in thread How to write a program to read stdin or command line? by princepawn

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