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#!/usr/bin/env perl

...

    push @{$result->{$keys[$_]}}, $values[$_] for 0 .. $#keys;
}

{ H1 => [1, 8, 9], H2 => [2, 7, 10], H3 => [3, 6], H4 => [4, 5] }

my @B_copy = @B;
...
while (@B_copy) {
    build_hash(\%result, \@A, \@B_copy, $reverse);
...

my @A = qw{H1 H2 H3 H4 H5};
my @B = 1 .. 23;

{
  H1 => [1, 10, 11, 20, 21],
...
  H4 => [4, 7, 14, 17],
  H5 => [5, 6, 15, 16],
}