A dehydrating agent acting as an hydrating agent?
Reaction with water is not always a hydration reaction.

A hydration reaction is one in which all of the atoms of the water molecule get added to the reactant to form a product:

$$\ce{MX + H2O -&gt; MH2XO}$$

$$\ce{C2H4 + H2O}\overset{\ce{H+}}{\longrightarrow}\ce{C2H6O}$$

$$\ce{P4O10 + 6H2O -&gt; 4H3PO4}$$

The reaction of water with phosphorous pentachloride is more properly a substitution or partial metathesis reaction. The hydrogen atoms in water are not incorporated into the product:

$$\ce{PCl5 + H2O -&gt; POCl3 + 2HCl}$$

This reaction is difficult to control because $\ce{POCl3}$ is reactive with water also, 
and under the right conditions can yield phosphoric acid or even $\ce{P4O10}$.

$$\ce{POCl3 + 3H2O -&gt; H3PO4 + 3HCl}$$
$$\ce{4POCl3 + 6H2O -&gt; P4O10 + 12HCl}$$

The reaction of $\ce{P4O10}$ and $\ce{PCl5}$ is thus also a partial metathesis. It is more obvious when you balance the equation. The oxygen and chlorine atoms are rearranging between the phosphorous atoms.

$$\ce{P4O10 + 6PCl5 -&gt; 10POCl3}$$