my %count=();  # A hash -it is bcos of of key/value pairs?

while (<$fh>){
  if (/amavis.*(Passed|Blocked) (CLEAN|SPAM|INFECTED|BANNED)/){
    ++$count{$2}; # Why ++$count? why {$2} here? is it bcos of (Passed|Blocked) ?
  } elsif (/amavis.*(BAD-HEADER)-/) {
    ++$count{$1}; # why {$1} here? is it bcos of (BAD-HEADER)
  } elsif (/reject/){
    ++$count{'REJECTED'}; #Hmm, Why {'REJECTED'}?
  }
}



##</code><code>##


my @data = map{ $count{$_} || 0 } qw(CLEAN SPAM INFECTED BANNED BAD-HEADER REJECTED);

Why map{ $count{$_} || 0 } ? 

qw(CLEAN SPAM INFECTED BANNED BAD-HEADER REJECTED) # here you create an array for count values?


##</code><code>##


my $data = GD::Graph::Data->new([
    ["Cleaned","Spam","Virus","Banned","BadHeader","Rejected"],
    [@data],
]) or die GD::Graph::Data->error; 

[@data] has count values coming from  line qw(CLEAN SPAM INFECTED BANNED BAD-HEADER REJECTED)