Keeping the default in the %par hash could be one way. Seems relatively clean to me...

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11145036
use warnings;

my %par = map { split ' ', $_, 2 } split /\n/, <<END;
margin-left defaults to margin-horizontal
margin-horizontal defaults to margin
margin 20
margin-right 3
margin-bottom defaults to margin-right
END

use Data::Dump 'dd'; dd 'dump of %par', \%par;

sub get
  {
  my $want = shift;
  $want = $1 while ($par{$want} // '') =~ /^defaults to (.+)/;
  return $par{$want} // 'not defined';
  }

for my $parameter ( sort 'margin-top', keys %par )
  {
  printf "%30s => %s\n", $parameter, get($parameter);
  }
[download]

Outputs:

(
  "dump of %par",
  {
    "margin"            => 20,
    "margin-bottom"     => "defaults to margin-right",
    "margin-horizontal" => "defaults to margin",
    "margin-left"       => "defaults to margin-horizontal",
    "margin-right"      => 3,
  },
)
                        margin => 20
                 margin-bottom => 3
             margin-horizontal => 20
                   margin-left => 20
                  margin-right => 3
                    margin-top => not defined
[download]

UPDATE: fixed typo in regex