use strict;
use warnings;

use Data::Dumper;

my %main = ( 'main1' => {'m1sec1'=> ['A','B','C'],},
             'main2' => {'m2sec1'=> ['D','E','F'],},);

my %seed = ( 'seed' => {'seed1'=> ['X','Y','Z']},);

foreach  my $mn ( keys %main )
{
    my $joined = $main{"$mn-join"} = {};

    foreach  my $msec ( sort keys %{$main{$mn}} )
    {
        my $store = $joined->{$msec} = [];

        foreach  my $valmain( @{$main{$mn}{$msec}}  )
        {
            foreach  my $sd ( keys %seed )
            {
                foreach  my $sdsub ( sort keys %{$seed{$sd}} )
                {
                    foreach  my $valseed ( @{$seed{$sd}{$sdsub}}  )
                    {
                            push @$store, $valmain.$valseed;
                    }
                }
            }
        }
    }
}

print Dumper \%main ;