very nice solutions, All!

Me? late to the party as usual, but my contribution:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my @array = qw(1 2 3 4 5 6 7 8 9 10 11 12);
my $size = @array;
my $count = int($size / 4);
my $counter = 1;
my $result = {};

for (0..2) {
  push @{$result->{$counter++}}, @array[0 ..$count-1];
  @array = @array[$count .. $#array];
}
push @{$result->{$counter}}, @array[0 ..$#array] if @array;
print Dumper($result);

__output__
$VAR1 = {
          '1' => [
                   '1',
                   '2',
                   '3'
                 ],
          '3' => [
                   '7',
                   '8',
                   '9'
                 ],
          '4' => [
                   '10',
                   '11',
                   '12'
                 ],
          '2' => [
                   '4',
                   '5',
                   '6'
                 ]
        };
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