You are telling the program that when you split your line on '::', you will get two variables. So the program gives you two variables, even if one or both of them are empty. Then when you try to use the variable $key, it complains because $key is "uninitialized."

This is Perl being helpful since if it did not warn you, you wouldn't know that $key is not defined and that therefore the next line is not going to accomplish what you think.

Just make the next statement conditional on $key and $value having been initialized and populated. How you check that depends on whether you are concerned that either or both of the variables are defined, or are non-empty, or meet some other requirement.

UPDATE: removed a bunch of examples that could have led OP in the wrong direction, thx Alexander...

If you only want to populate your hash when both $key and $value contain text, you could do something like:

  my %data_ids;
  my $count = 0;
  while ( # loop through lines from file ) {
    $count++;
    chomp $readline;
    my ($key , $value) = (split /::/, $readline);
    if (defined $key and $key ne '' and defined $value and $value ne '
+') {
      $data_ids{$key} = $value;
    } else {
      warn "No key-value pair from line $count: $readline\n";
    }
  } # end loop
[download]

Also, you don't say whether you have been given this config file or whether you are building it yourself. If you have control over the file, consider using https://metacpan.org/pod/Config::Tiny which will take a config file in standard .ini format ( key=value ) and return the config to you in a hashref - with no blank lines!