Question

What does reading from <> do in Perl? For example, what will the following do?

print for(<>);

Answer 1

The so-called diamond operator (<>) reads line-by-line (in scalar context) from STDIN or the filename(s) specified as command-line arguments.

From perldoc perlop:

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.

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In list context, <> returns all lines, with each line stored as an element in the list.

This means that print for <>; will do the same thing as print while <>;, albeit with more memory.