A glitch with importing a list of lists of polynomials

Would anybody know how to import a list of lists of polynomials which I exported earlier?

d = {{3*t^2 + 2*t^(-1), 5*t + 6*t^(-2) + 7*t^(-4)}, {7*t^2 + 8*t^(-1), 9*t + 10*t^(-2)}}  Export["mydata.dat",d]  Import["mydata.dat"]  Out[]={{"2/t", "+", "3*t^2", "7/t^4", "+", "6/t^2", "+", "5*t"}, {"8/t", "+", "7*t^2", "10/t^2", "+", "9*t"}}  imp=Import["mydata.dat", "List"] Out[]={2/t + 3*t^2  7/t^4 + 6/t^2 + 5*t, 8/t + 7*t^2    10/t^2 + 9*t} imp[[1]] Out[]= 2/t + 3*t^2  7/t^4 + 6/t^2 + 5*t imp[[1]][[1]]  error: Part::partd: Part specification 2/t + 3*t^2  7/t^4 + 6/t^2 + 5*t[[1]] is longer than depth of object. Head @ imp[[1]] Out[]=String 

Exporting as "List" does not help.