You seem to believe that the OP wants to read manual input from STDIN², but - as kcott noted - the OP said files.

Please note that an empty diamond <> defaults to <ARGV> .

And ARGV only defaults to "-" ° if the array @ARGV is empty.

So if you want to read from STDIN, better say so explicitly by writing <STDIN> .

from perlop

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. The loop 
while (<>) {
    ...                     # code for each line
}

[download]
is equivalent to the following Perl-like pseudo code: 
unshift(@ARGV, '-') unless @ARGV;
while ($ARGV = shift) {
    open(ARGV, $ARGV);
    while (<ARGV>) {
        ...         # code for each line
    }
}

[download]
except that it isn't so cumbersome to say, and will actually work. It really does shift the @ARGV array and put the current filename into the $ARGV variable. It also uses filehandle ARGV internally. <> is just a synonym for <ARGV>, which is magical. (The pseudo code above doesn't work because it treats <ARGV> as non-magical.)

NB: took me a while to find this, IMHO this should be at least mentioned/referenced in @ARGV

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

°) IIRC "-" is just an alias for STDIN which can be passed literally as argument at the command line

²) STDIN is not necessarily the keyboard, it depends how the program was started

In reply to Re^3: $_ not set in while <> by LanX
in thread $_ not set in while <> by pidloop

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