See Those fork()ing flock()ers... for some information on this very issue.

Also, may I suggest the following change:

my @custom = (
 qr /postmaster/ ,
 qr /webmaster/
);

 ...

    foreach $x (@custom){
              if ($add =~ $x ){
                  $FH = "BADADDR";
                  writeaddr(); #address is bad
                  $pm->finish;
              }
          }
[download]

By compiling your regular expressions only once before you fork, you may save some time. Especially if you have a very long list of e-mails.