I think jdporter covered things pretty well. One more trick you might want to know: the "-t" function (one of the things described in perlfunc, particularly in the section that appears when you run "perldoc -f -X").

The "-t", used without any parameter, will return true if your current STDIN is coming from a terminal (as opposed to coming from a pipeline or input redirection). Here's how you might use it:

#!/usr/bin/perl

use strict;
use warnings;

my @files;
if ( @ARGV ) {
    @files = grep { -f } @ARGV;
}
elsif ( -t ) { # true if STDIN is a tty (not a pipe)
    die "Usage:  $0 file [file2 ...]\n   or: ls | $0\n";
}
else {  # get here when STDIN is connected to a pipe
    @files = <>;
    chomp @files;
}

# ... do stuff with @files
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Try running that without args and without any input pipe or redirection, and you'll see the "Usage:" message.

Update: <pedantic> I felt compelled to point out that the particular design you want, IMO, is not really a "unix-style filter". For the great majority unix-style filters, the following types of command lines are (more or less) equivalent:

cat file1 | tool
tool file1
tool < file1

cat file1 file2 ... | tool
tool file1 file2 ...
# (no equivalent using "<" in this case)
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(The first example, of course, demonstrates "useless use of cat", but I included it for pedagogical completeness). </pedantic>