Just to deal with the part of reading command line arguements, as was pointed out, they are stored in @ARGV, i.e.

...
if ( @ARGV != 2 ) {
   # check that you have the proper number of args
   print "usage: $0 pattern infile_name\n";
   exit;
}

my ($pattern, $infile_name) = @ARGV;
...
[download]

I generally like to check that i have the proper command line args before i do anything else, although this checking is often more complicated.