Are all the entries in the fifth column of my $mio=$array_with_all_fields[4] unique ?

If the answer is "yes" then your hash can be re-written as has been suggested to you (i.e.  %go_accession_hash{$mio}=' ';). If the answer is "no" then you might want to consider using an advanced data structure that is more appropriate to capture each variation associated with $mio.

In order to help you understand the problem and solution better It will be more useful to actually show a representative example of how your data look like and what you want to achieve from reading it in.