The issue is not with the "importdata" function.
The below command which you used to concatenate the two char arrays should throw a Dimension mismatch error.
>> prf_string(1,1) = strcat(prf_name, prf_ext);
I do not see the need to use array indexing here.
>> prf_string = strcat(prf_name, prf_ext);
I believe that would solve your issue.
PS: Refer this link on how to work with character arrays & strings.