#!/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

##</code><code>##

cat file1 | tool
tool file1
tool < file1

cat file1 file2 ... | tool
tool file1 file2 ...
# (no equivalent using "<" in this case)