use POSIX qw/setuid setsid/;

BEGIN {
 setuid scalar getpwnam 'qf3' or die $!;
 # Fork.
 my $pidFile = '/some/path/pid';
 my $pid = fork;
 if ($pid) # parent: save PID
 {
  open PIDFILE, ">$pidFile" or die "can't open $pidFile: $!\n";
  print PIDFILE $pid;
  close PIDFILE;
  exit 0;
 }
}

print "USER: ". getpwuid($<) . "\n";