As previously mentioned, multi-tasking won't help. This prorgram is I/O-bound, and merging the results of the threads would be as expensive as building the results in the first place.

I just wanted to provide a cleaned up version of your code (with a few micro-optimizations).

#!/usr/bin/perl
use strict;
use warnings;
use feature qw( say );

use Sort::Naturally qw( nsort );

local $/ = '';  # Paragraph mode reads until a blank line.

my %grouped;
while (<>) {
   my @lines = split /\n/, $_;

   for (@lines[1..$#lines]) {
      next if $_ eq '--';   # Omit if rarely true.

      ++$grouped{"D$1"}{$2} while /\bD\*([^*]*)\*([^*]*)\*D\b/g;
   }
}

for my $k (nsort keys %grouped) {
   say "$k=>" . join("|", keys(%{ $grouped{$k} });
}
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