One definition that I've often heard is that scripts are interpreted and applications are compiled. Of course this works in Perl's favour as Perl programs are compiled, even if it looks like they're interpreted.

Update: Should probably make it clear that this isn't a categorisation that I agree with (for many reasons including the ones mentioned by mikeB below. I jsut mention it as the most common explaination that I've heard.

"The first rule of Perl club is you don't talk about Perl club."