Isn't it a typical use case of tied hash ? This code for example should do exactly what you want, without requiring any get or set function:

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);
}
[download]

You can test it by adding this code at the end of the file for example:

package main;

my %par;
tie(%par,'ParamsHash');

print "Initial values:\n";
printSomeParams();

print "Setting 'margin' to 4:\n";
$par{margin}=4;
printSomeParams();

print "Setting 'margin-horizontal' to 8:\n";
$par{'margin-horizontal'}=8;
printSomeParams();

print "Setting 'margin-left' to 3:\n";
$par{'margin-left'}=3;
printSomeParams();

sub printSomeParams {
  map {printParam($_)} (qw'
                        margin
                        margin-horizontal
                        margin-left
                        margin-right
                        ')
}

sub printParam { print "  $_[0] = $par{$_[0]}\n" }
[download]

This should give you following result:

Initial values:
  margin = a
  margin-horizontal = a
  margin-left = a
  margin-right = a
Setting 'margin' to 4:
  margin = 4
  margin-horizontal = 4
  margin-left = 4
  margin-right = 4
Setting 'margin-horizontal' to 8:
  margin = 4
  margin-horizontal = 8
  margin-left = 8
  margin-right = 8
Setting 'margin-left' to 3:
  margin = 4
  margin-horizontal = 8
  margin-left = 3
  margin-right = 8
[download]

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