The message you are seeing is a warning. It arises only if you have look_through_file declared with a prototype; for example:

sub look_through_file($$)
{
   ...
[download]

There are three ways to deal with this:

1. Move the definition of sub look_through_file to before the code where it is called (the while loop) — as recommended by daxim, above.
2. Leave the definition where it is, but add a declaration before the calling code:

   sub look_through_file($$);
   [download]

3. Omit the prototype altogether. Unless you have a really good reason for including it, a prototype is usually a bad idea anyway.

See Prototypes. Hope that helps,