@DISKD = <inputfile>;
foreach $domain (@domainlist) {
  @value = grep { /^\Q$domain\E\b/ } @DISKD;
  for $value (@value) {
     @data = split /\s+/, $value;
     $sum += $data[$#data];
  }
}

##</code><code>##

# Calculate the sum for every domain.
@DISKD = <inputfile>;
foreach (@DISKD) {
   @data = split(/\s+/, $_);
   $sum{$data[0]} += $data[1];
}

# Filter out the domains we don't want.
# We can even skip this step if we don't care
# if %sum has more domains than @domainlist.
%domainlist = map { $_ => 1 } @domainlist;
foreach $domain (keys(%sum)) {
   delete $sum{$domain} unless $domainlist{$domain};
}