You can either have the script read from STDIN, or supply a filename as an argument, i.e.:

$ perl script.pl data.dat
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or

$ cat data.dat | perl script.pl
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Such is the magic of the angle bracket operator when used with a null filehandle. Quoting perlop:

The null filehandle <> is special: it can be used to emulate the behavior of sed and awk, and any other Unix filter program that takes a list of filenames, doing the same to each line of input from all of them. 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
    }
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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
    }
    }
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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.)

Hope this helps!