This is fairly straightforward - you might want to use Super Search for other examples.

Commandline arguments are available in the array @ARGV

Try something like this:

#!/usr/bin/perl -w
use strict;

my $in_file = shift @ARGV;

open(INFILE, "$in_file") or die "Can't open input file!: $!\n";

while (<INFILE>) {
  # do stuff with each line of INFILE until
  # there are no more lines to process
}

close(INFILE);
[download]

Hope that helps.

BazB.