use strict;
use warnings;

use Data::Dumper;


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

my @seed = ( 'X','Y','Z' );


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

        foreach my $val1 ( @{$main{$mn}{$msec}} )
        {
            foreach my $val2 ( @seed )
            {
                foreach my $val3 ( @seed )
                {
                    push(@$store, "$val1$val2$val3");
                }
            }
        }
    }
}


# Show updated %main.

print(Dumper(\%main));