require Math::Random::MT;;

cmpthese -1,{a=>q[my $a; $a=rand() for 1..1e6],b=>q[my $a; $a=Math::Random::MT::rand() for 1..1e6] };;
            (warning: too few iterations for a reliable count)

  s/iter     b     a
b   1.58    --  -93%
a  0.105 1407%    --