use strict;
use warnings;

use Data::Dumper;


# Inputs.

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

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

my $output_length = 3;


# Create a work area since we can't
# modify the existing content of %main.

my %work;

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


# Repeatedly multiply matrices (in place).

$output_length--;

while ($output_length--)
{
    foreach my $mn ( keys %work )
    {
        foreach my $msec ( sort keys %{$work{$mn}} )
        {
            my @store;

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

            $work{$mn}{$msec} = \@store;
        }
    }
}


# Merge output with %main.

%main = (%main, %work);


# Show updated %main.

print(Dumper(\%main));