use Data::Dumper;
...
# after some assignment like
    $metrics{$product}{$date} = $day;
    print STDERR Dumper \%metrics;
    $metrics{$product}{$date}{$hour}{$user} = $val;
    print STDERR Dumper \%metrics;

##</code><code>##

C:\usr\local\share\PassThru\perl>perl -MData::Dumper -le "$metrics{product}{today} = 'Wed'; print STDERR Dumper \%metrics"
$VAR1 = {
          'product' => {
                         'today' => 'Wed'
                       }
        };