Is this what you are looking for? 
cat test

1;2; 3; 4; 5 ; 5
2;2;4; 4; 6; 7
3;1;2; 3;4;5
4;1;1;1;1; 1

[download]


perl -MData::Dumper -le 'my %hash;
while(<>) {
my ($field1,$field4)=(split /\s*;\s*/)[0,3];
push @{$hash{$field4}},$field1 }
 print Dumper(\%hash) ' test
$VAR1 = {
          '1' => [
                   '4'
                 ],
          '4' => [
                   '1',
                   '2'
                 ],
          '3' => [
                   '3'
                 ]
        };

[download]
Update: about the actual problem in your posted code, I think you will need to refer Autovivification in Perl.

In reply to Re: A Hash that is giving me the ####s by suhailck
in thread A Hash that is giving me the ####s by hoffy

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