It doesn't make a difference, but if you want to write something after the variable, it's treated literally unless there's an \E. That's a common mistake I made, so I always haven an \E for every \Q.

Example:

$ perl -Mre=debug -e 'my $x= "*"; /\Q$x.+/'
Compiling REx "\*\.\+"
Final program:
   1: EXACT <*.+> (3)
   3: END (0)
anchored "*.+" at 0 (checking anchored isall) minlen 3 
Freeing REx: "\*\.\+"
[download]

You see that the .+ is also treated literally.