How did Hiruzen Sarutobi summon enma when he was a reanimation?
I can recall two instances where Shinobi who are under the influence of Impure World Reincarnation are shown to bleed while when performing a justu that requires blood or causes them to bleed.

While those reincarnated by Impure World Reincarnation do not bleed, it would appear reincarnated Shinobi do bleed as long as the injury is self applied (speculative and don't have a reference, somebody correct me if I am wrong on this). 

The first instance is the Summoning Justu


  Reincarnated bodies do not bleed, except in those cases where blood is needed to perform the Summoning Technique


This is in reference to Naruto Chapter 548, when we see the reincarnated 2nd Mizukage biting his thumb to perform the Summoning Justu against the Allied Shinobi Forces



The second instance is using the Mangekyou Sharingan


  Other side-effects may still manifest, as when Itachi Uchiha's eyes bleed from using his Mangekyō Sharingan, but he is not inhibited by it.


This is in reference to Naruto Chapter 550, when Naruto and Killer B are fighting against the reincarnated Nagato and Itachi. Itachi uses the Mangekyō Sharingan, causing his eyes to bleed