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";
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USER will be root, but when I do a ps -ef it does:

toadi      20510     1 14 04:56 pts/0    00:00:01 /usr/bin/perl /some/
+path/daemon
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