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

In reply to Re: Deferring variables by tybalt89
in thread Deferring variables by Chuma

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.