use warnings;
use strict;

package ParamsHash;
require Tie::Hash;
our @ISA='Tie::StdHash';

use constant { UNDEF_VAL => 'a' };

# configure your parameter inheritance here:
our @GLOBAL_PARAMS=(qw'margin padding');
our %SUB_PARAMS=(horizontal => [qw'left right'],
                 vertical => [qw'top bottom']);

my %INHERIT;
foreach my $param (@GLOBAL_PARAMS) {
  foreach my $subp (keys %SUB_PARAMS) {
    map {$INHERIT{"$param-$_"}="$param-$subp"} @{$SUB_PARAMS{$subp}};
    $INHERIT{"$param-$subp"}=$param;
  }
}

sub FETCH {
  my ($this,$key)=@_;
  my $val=$this->{$key};
  return $val if(defined $val && $val ne UNDEF_VAL);
  my $inherited=$INHERIT{$key};
  return UNDEF_VAL unless(defined $inherited);
  return $this->FETCH($inherited);
}