Sorry, I initially misunderstood your question, but reading some other replies helped.

(my main processor is (st)ill suffering from a virus attack ;-)

I tried aliasing, but no joy, after the first level you have far too much trouble maintaining.

I tried refs on hash elements, it worked but was a bit too fickle, if the data structure changed.

I tried just one data structure, denoting the links with special values like \"margin" , but again too much trouble maintaining such a mangled structure.

My suggestion is to keep it simple, an additional hash %def maps to the default and you use a simple recursive get-function.

 
use v5.12;
use warnings;
use Data::Dump qw/pp dd/;

my %def =
  (
   'margin-left'       => 'margin-horizontal',
   'margin-horizontal' => 'margin',
   # ...
  );

my %par =
  (
   'margin' => 42,
  );



sub get {
    my $key = shift 
       // return  "UNDEFINED";               # or whatever error mecha
+nism you prefer
    $par{$key} // get( $def{$key} )
}


say get("margin-left");                      # 42 

$par{'margin-horizontal'} = 666;

say get("margin-left");                      # 666

undef $par{'margin-horizontal'};

say get("margin-left");                      # 42  

say get("TYPO");                             # UNDEFINED
[download]

please note that I also preferred undef over "a"

Otherwise you can easily change the get code with a ternary

$par{$key} ne "a" ? $par{$key} : get( $def{$key} );