One way to ease the brain ache is to use Data::Dumper to visualise your data structures.

knoppix@Microknoppix:~$ perl -MData::Dumper -Mstrict -wE '
> my %rep  = ( one => 123, two => 456 );
> my %comb = ( one => 987, two => 654 );
> my %new  = map { $_, [ $rep{ $_ }, $comb{ $_ } ] } keys %rep;
> print Data::Dumper->Dumpxs(
>    [ \ %rep, \ %comb, \ %new ],
>    [ qw{ *rep *comb *new } ]
>   );
> say q{-} x 30;
> push @{ $new{ two } }, 888;
> $new{ three } = { four => 444, five => 555 };
> print Data::Dumper->Dumpxs( [ \ %new ], [ qw{ *new } ] );'
%rep = (
         'one' => 123,
         'two' => 456
       );
%comb = (
          'one' => 987,
          'two' => 654
        );
%new = (
         'one' => [
                    123,
                    987
                  ],
         'two' => [
                    456,
                    654
                  ]
       );
------------------------------
%new = (
         'three' => {
                      'five' => 555,
                      'four' => 444
                    },
         'one' => [
                    123,
                    987
                  ],
         'two' => [
                    456,
                    654,
                    888
                  ]
       );
knoppix@Microknoppix:~$
[download]

I hope this is helpful.