use strict;
use warnings;

use Email::Address ();

my @fields;

{
   open(my $fh_in, '<', ...)
      or die("Unable to open input file: $!\n");

   # The input is a binary file.
   binmode($fh_in);

   for (;;) {
      my $buf;

      # Obtain the length of the field.
      read($fh_in, $buf='', 1)
         or last;

      $len = ord($buf);

      # Obtain the field.
      read($fh_in, $buf='', $len)
         or die("Bad input file\n");

      push(@fields, $buf);
   }
}

@fields % 3 == 0
   or die("Bad input file\n");

# Get every third field, starting with the first:
my @email_addrs = do {
   my $n = 3;
   my $t = $n-1;
   grep !($t=($t+1)%$n), @fields
};

# Convert to Email::Address objects.
# Group by host.
my %grouped;
foreach my $email_addr (@email_addrs) {
   foreach my $o (Email::Address->parse($email_addr)) {
      $host = lc($o->host());
      push(@{$grouped{$host}}, $o);
   }
}

# Determine the most common hosts.
my @highest = sort { @{$grouped{$b}} <=> @{$grouped{$a}} } keys %group
+ed;

# Filter out unwanted. Keep ties.
if (@higest >= 5) {
   my $min = @{$grouped{$highest[4]}};
   @highest = grep { @{$grouped{$_}} >= $min } @highest;
}

# Print results.
foreach my $host (@highest) {
   my $count = @{$grouped{$host}};
   print("$host ($count)\n")
}
[download]

Untested.