use Fcntl qw(:flock);




my $file = '/home/X/public_html/filelock';
   my $SEMAPHORE = "$file.lck";

   open(LOCK, ">$SEMAPHORE") || die $!;
    flock(LOCK, LOCK_EX);

    unless (-e "file.html") {
     open (HTMLFILE,">file.html");
     require 'html_head_page.pl';
    close (HTMLFILE);	
   close(LOCK); 
    unlink("$SEMAPHORE");  
   }