use CGI;
use POSIX qw(setsid);
use Fcntl qw(:flock);
    ...
 my $Session = ...
    ... without session ... deny access

 my $SessionFile = "/tmp/$Session";

  # Touch control file
  unless ( -f $SessionFile ) { open(C, ">$SessionFile"); close(C) }

  unless ( open(F, "+< $SessionFile") ) {
    print $q->h1("Session control failure");
    exit 1;
  }

  unless ( flock(F,  LOCK_EX | LOCK_NB) ) {
    print $q->h1("Session locked, is running ?");
    exit 1;
  }

  ... Daemonize like process ...

 my $pid;
   open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
   open STDOUT, '>/dev/null'
            or die "Can't write to /dev/null: $!";
   unless ( defined($pid = fork ) ) {
    print $q->h1("Can't fork: $!");
    exit 1;
   }

   if ( $pid ) { # I'm parent... good by to User
    print "Good Bye,<br>Wait for mail after process...";
    exit 1;
   }

  #-- dissociate Apache CGI shell
  unless ( setsid ) {
    warn "Can't start a new session: $!";
    ... mail to user warning failure ...
    exit 1;
  }

  ... do your process ...

  ... see merlyn's sugestions about sendmail ...

  flock(F, LOCK_UN );
  close(F);
  exit 1;
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