I looked around for this and got frustrated with how hard it was. Then I finally found the correct recipie, and I post it here. Use this code when you write a perl server that needs to do some root stuff and then give up root privilages. The group name and user name are probably in your config setup somehow.
sub switchUsers { my ($uname,$gname)=@_;
 eval('getgrnam(root);getpwnam(root);'); 
 if($@) {
  # windows pukes "unimplemented" for these -- just skip it
  mlog("Warning: uname and/or gname are set ($uname,$gname) but getgrn
+am / getpwnam give errors");
  return;
 }
 $>=0;
 if($> != 0) {
  mlog("requested to switch to user/group '$uname/$gname' but cannot s
+et effective uid to 0 -- quitting; uid is $>");
  die "requested to switch to user/group '$uname/$gname' but cannot se
+t effective uid to 0 -- quitting; uid is $>\n";
 }
 $<=0;
 if($gname) {
  my $gid = getgrnam($gname);
  if(defined $gid) {
   $)="$gid $gid";
   if($)+0==$gid) {
    mlog("Switched effective gid to $gid ($gname)");
   } else {
    mlog("Failed to switch effective gid to $gid ($gname) -- effective
+ gid=$) -- quitting");
    die "Failed to switch effective gid to $gid ($gname) -- effective 
+gid=$) -- quitting";
   }
   $(="$gid $gid";
   if($(+0==$gid) {
    mlog("Switched real gid to $gid ($gname)");
   } else {
    mlog("Failed to switch real gid to $gid ($gname) -- real uid=$(");
   }
  } else {
   mlog("Could not find gid for group '$gname' -- not switching effect
+ive gid -- quitting");
   die "Could not find gid for group '$gname' -- not switching effecti
+ve gid -- quitting";
  }
 }
 if($uname) {
  my $uid = getpwnam($uname);
  if(defined $uid) {
   # do it both ways so linux and bsd are happy
   $<=$uid; $>=$uid; $<=$uid; $>=$uid;
   if($>==$uid) {
    mlog("Switched effective uid to $uid ($uname)");
   } else {
    mlog("Failed to switch effective uid to $uid ($uname) -- real uid=
+$< -- quitting");
    die "Failed to switch effective uid to $uid ($uname) -- real uid=$
+< -- quitting";
   }
   if($<==$uid) {
    mlog("Switched real uid to $uid ($uname)");
   } else {
    mlog("Failed to switch real uid to $uid ($uname) -- real uid=$<");
   }
  } else {
   mlog("Could not find uid for user '$uname' -- not switching effecti
+ve uid -- quitting");
   die "Could not find uid for user '$uname' -- not switching effectiv
+e uid -- quitting";
  }
 }
}

sub mlog {print @_,"\n";}

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In reply to relinquish root privilages by jhanna

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