I'll address the basics, since there's a lot of work/learning to be done there.

• I opened IN for reading. You had it open for writting.
• I initilized (by localizing) @my_bucket to allow get_country to be called more than once.
• I added error checking.
• I switched to the safer 3 arg open.
• I localized IN.
• Now that IN is localized, close IN; is optional. I removed it.
• I used while (<IN>) instead of loading the entire file into memory. That was needlessly inefficient.

sub get_country {
   my $data = shift;
   
   open(local *IN, '<', 'country.lis')
      or die("Unable to open list of countries country.lis: $!\n");

   my @my_bucket;
   while (defined(my $c = <IN>)) {
      ...
   }

   return \@my_bucket;
}
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