I found the answer here: http://archive.develooper.com/perl5-changes@perl.org/msg06044.html

Linux and BSD treat this slightly different, and the safe way is like this:

$< = $id; # real uid
$> = $id; # effective uid
$< = $id; # real uid
$> = $id; # effective uid
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If you do it twice, beginning with the real then everybody is happy. So the final answer to my question is here:

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 get
+grnam / getpwnam give errors");
  return;
 }
 $>=0;
 if($> != 0) {
  mlog('',"requested to switch to user/group '$uname/$gname' but canno
+t set 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) -- effect
+ive 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 eff
+ective 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 u
+id=$< -- 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 effe
+ctive uid -- quitting");
   die "Could not find uid for user '$uname' -- not switching effectiv
+e uid -- quitting";
  }
 }
}
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