$ perl script.pl data.dat

##</code><code>##

$ cat data.dat | perl script.pl

##</code><code>##

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

##</code><code>##

    unshift(@ARGV, '-') unless @ARGV;
    while ($ARGV = shift) {
	open(ARGV, $ARGV);
	while (<ARGV>) {
	    ...		# code for each line
	}
    }