How do I do this? read __DATA__ then seek to 0 then write over it?

Yes, just open up the current script and write to it like you would any other file e.g

use Fatal qw/ open seek /;

open(my $d => '+<', $0);

seek $d => 0, 2;

print $d @ARGV;

__DATA__
foo
bar
[download]

This will append @ARGV to the __DATA__ section of the currently executing script. Of course modify the seek as you see fit and if you're going to completely overwrite it then remember it's position with a quick tell.
HTH

_________
broquaint