use File::Tail;

my $tail = File::Tail->new("/some/log/file");

my @alert;
while (defined(my $line = $file->read)) {
   if ($line =~ /^=\+=/) {
       process_alert(\@alert);
       @alert = ();
   } else {
       push @alert, $line;
   }
}

process_alert(\@alert) if @alert;