Yes, 'interpolation' as it is known is one of the features of Perl (probably stolen from the Unix shell) that other languages are most jealous of. I never enjoy the concatentation process in say JavaScript and don't even get me started on Java :-)

By the way, you only need to use the braces round the variable name when its followed by valid name characters, eg: you can simply say something like "$dir/$user.txt" (although a more portable approach would be to use File::Spec).