It's a hack but it gives you a hash mapping each name to it's index:

(see Dynamically inspecting Regex OP-Codes at runtime? for an explanation)

use strict;
use warnings;
use Data::Dump;

my $a=qr/(?<C>1)(?<D>2)(3)(?<A>4)/;

my $parsing=parse_regex($a);

# parse lines like '   1: OPEN1 'C' (3)'
my %named_captures =
  ($parsing =~ /^
                \s{1,3}\d{1,3}:\s+        # token number
                OPEN(\d)[ ]'(\w+)'        # group(nr) 'name'
                [ ]\(\d+\)                # next token
                $
               /xgm);

my %index_named_capture = reverse %named_captures;



dd \%index_named_capture;            # OUTPUT     { A => 4, C => 1, D 
+=> 2 }



sub parse_regex {
  my $regex=shift;

  my $re_compilation;

  # First, save away STDERR
  open my $SAVEERR, ">&STDERR";
  close STDERR;
  open STDERR, ">", \$re_compilation or die "What the hell?\n";

  # Now dynamically recompile a new regex, saving debug_info to $re_co
+mpilation
  eval <<'_code_';
use re 'debug';
my $b=qr/$regex(?:)/;
_code_


  # Now close and restore STDERR to original condition.
  close STDERR;
  open STDERR, ">&", $SAVEERR;

  return $re_compilation;
}
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UPDATES

corrected typo in code