Consult perlfunc, which contains this excerpt from eval:

With an "eval", you should be especially careful to remember what's being looked at when:

eval $x;            # CASE 1
eval "$x";          # CASE 2

eval '$x';          # CASE 3
eval { $x };        # CASE 4

eval "\$$x++";      # CASE 5
$$x++;              # CASE 6
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Cases 1 and 2 above behave identically: they run the code contained in the variable $x. (Although case 2 has misleading double quotes making the reader wonder what else might be happening (nothing is).) Cases 3 and 4 likewise behave in the same way: they run the code '$x', which does nothing but return the value of $x. (Case 4 is preferred for purely visual reasons, but it also has the advantage of compiling at compile-time instead of at run-time.) Case 5 is a place where normally you would like to use double quotes, except that in this particular situation, you can just use symbolic references instead, as in case 6.