Definition: In programming terms, data type refers to a specific classification (or grouping) of information.
Many languages (C, C++, Java) require that you explicitly state what kind of type your data fits in. These languages are called strongly typed. This way, the computer knows specifically what kind of information you are feeding it - whole numbers, decimals, text, etc.
Perl is a loosely typed programming language. In the case of Perl, the term data type actually refers to a broader grouping of information. At it heart, Perl really only has a single data type - the scalar - but most people expand this to lists and hashes as well.