Rate  ig1  ig2   gf
ig1  8012/s   --  -0% -21%
ig2  8013/s   0%   -- -21%
gf  10181/s  27%  27%   --

##</code><code>##

use strict;
use warnings;
use Data::Dumper;
use Benchmark qw{cmpthese};

my %nhash;

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

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


print cmpthese (-5,
    {
    gf  =>   'gf ()',
    ig1 =>   'ig1 ()',
    ig2 =>   'ig1 ()',
    }
    );


sub gf
{
my @main1 = @{$main{'main1'}{'m1sec1'}};
my @main2 = @{$main{'main2'}{'m2sec1'}};
my @seed_a = @{$seed{'seed'}{'seed1'}};
my @m1sec1;
my @m2sec1;

for my $s (@seed_a)
    {
    push @m1sec1, "$_$s" for (@main1);
    push @m2sec1, "$_$s" for (@main2);
    }

my %gfmain = %main;
$gfmain{'main-join1'} = {'m1sec1' => [@m1sec1]};
$gfmain{'main-join2'} = {'m2sec1' => [@m2sec1]};
}


sub ig1
{
my %joined;

foreach  my $sd ( keys %seed )
{
    foreach  my $sdsub ( sort keys %{$seed{$sd}} )
    {
        foreach  my $valseed ( @{$seed{$sd}{$sdsub}}  )
        {
            foreach  my $mn ( keys %main )
            {
                foreach  my $msec ( sort keys %{$main{$mn}} )
                {
                    # Create hash if it doesn't exist.
                    $joined{$mn."-join"} ||= {};
                   
                    # Create array if it doesn't exist.
                    $joined{$mn."-join"}{$msec} ||= [];
                   
                    my $store = $joined{$mn."-join"}{$msec};


                    foreach  my $valmain( @{$main{$mn}{$msec}}  )
                    {
                        push @$store, $valmain.$valseed;
                    }
                }
            }
        }
    }
}

# Combine inputs and outputs.
my %ig1main = (%main, %joined);
}


sub ig2
{
my %ig2main = %main;
my @seed;  # Flattened %seed.

foreach  my $sd ( keys %seed )
{
    foreach  my $sdsub ( sort keys %{$seed{$sd}} )
    {
        push @seed, @{$seed{$sd}{$sdsub}};
    }
}


foreach  my $mn ( keys %ig2main )
{
    my $joined = $ig2main{"$mn-join"} = {};

    foreach  my $msec ( sort keys %{$ig2main{$mn}} )
    {
        my $store = $joined->{$msec} = [];

        foreach  my $valmain( @{$ig2main{$mn}{$msec}}  )
        {
            foreach  my $valseed ( @seed  )
            {
                push @$store, $valmain.$valseed;
            }
        }
    }
}
}