Try calling GetX and GetY on the mob returned by GetMobByNPCTypeID instead of on the ID returned by GetID.
Code:
my $revoltldrID = 189490;
my $ldrMob = $entity_list->GetMobByNpcTypeID($revoltldrID);
my $ldrX = $ldrMob->GetX();
my $ldrY = $ldrMob->GetY();