#!/bin/perl5

use strict;
use warnings;
use Data::Dumper;

my $distri;

$distri = { # hash ref
  one   => [qw{1 2 3}], # array ref
  two   => [qw{3 4 5}],
  three => [qw{6 7 8}],
};

my ($aref);

my $key_new = 'four';
$aref = [qw{9 10 11}];
$distri = update_hash($distri, $key_new, $aref);

my $key_existing = 'one';
$aref = [qw{12 13 14}];
$distri = update_hash($distri, $key_existing, $aref);

print Dumper $distri;


sub update_hash {
  
  # unpack and copy the arguments
  my ($href, $key, $aref) = @_;
  
  # key will be created if it doesn't exist
  # (autovivified)
  push @{$href->{$key}}, @{$aref}; # append
  # $href->{$key} = $aref;         # overwrite
  
  # return the copy
  # (leaving the original intact)
  return $href;
  
}