Hi tty1x,
A few snapshot observations and 2 'kobo' suggestion:

• A number of solution previously given to you actually work, only that some may not preserve the order of data.
• This does work for me, which is not far from your OP, except that I have to create the variable $match.

  use warnings;
  use strict;
  
  my $match = <<'EOF';
  dddd@fgg.com
  cccc@fgg.com
  eeee@fgg.com
  dddd@fgg.com
  
  EOF
  
  my %seen;
  my @r;
  
  foreach my $v ( split /\n/, $match ) {
      unless ( $seen{$v}++ ) {
          push @r, $v;
      }
  }
  $" = "\n";
  print "@r";
  [download]

• Please, as much as possible try not to use the variable $a, since function sort uses it