You probably need 
my %gp = (
  ...
  Gi => \@gi, # an array ref
  Gt => \@gt,
);

[download]
Hash values can only be scalars. An array reference is a scalar so we can use that.

In your while loop you will first need to test if it is an array ref and, if so, dereference it. Something like 

  ...
  $j = $gp{$i};
  my @values;
  if (ref $j eq 'ARRAY'){
    @values = @{$gp{$i}};
  }
  else {
    @values = ($j);
  }
  for $val (@values){
  ...

[download]
(not tested)
update: fixed typo

In reply to Re: How to improve my code? main concern:array as hash element by wfsp
in thread How to improve my code? main concern:array as hash element by xargon

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