As ikegami pointed out to me yesterday, the sub will only be compiled once, so it's only the assignment to $SIG{__WARN__} that will be duplicated. You could avoid that by only setting it if it's not already set:

while (my $line = <LOG>) {
 my (@entry) = split(/\s+/, $line);
 my (@edate) = Parse_Date(substr($entry[0],0,10));
 my ($http_code) = substr($entry[3], -3);
 my ($url) = "$1$2"
      if ($entry[6] =~ m|^http://.*v1=(.*)&v2=(.*)&v3=.*$|);

 $SIG{'__WARN__'} ||= sub {
   print "WARNING: ". $_[0] ."\n".
         " http_code: $http_code\n".
         " url: $url\n".
         " edate[0]: $edate[0]\n".
         " edate[1]: $edate[1]\n".
         " edate[2]: $edate[2]\n".
         " edate[3]: $edate[3]\n"
     if ($_[0] =~ /^Use of uninitialized value in hash element.*$/);
 };

 $STATUS{$http_code}{$url}{$edate[0]}{$edate[1]}{$edate[2]}{$edate[3]}
+++;
}
[download]