use strict;
use Scalar::Util qw/reftype/;

my %hash_to_do

sub do_hash {
  my $hash_ref = shift;

  foreach my $key (keys %$hash_ref) {
    my $value_type = reftype $hash_ref->{$key};

    if ($value_type eq 'HASH') {
      do_hash($hash_ref->{$key});
    }
    else {
      # do something else
    }
  }
}