The reason that most people say you shouldn't use .* is because it can match nothing (or everything) so matching just .* is pointless because it will match everything (including nothing) However, if you were looking for "hi" some ammount of text and then "there" you could use:
$line =~ /hi.*there/;
and it would match. Of course it's greedy and might not be exactly what you wanted but there are times when it is needed. However, it is overused a lot and usually something better can be used.
To answer your question though, /.* doesn't grab everything because it has required stuff after that. If you try and match /.*some text/ It has to find "some text" or it will fail. However, if you try and match something like: /.*\d?/ it could match nothing since the \d is optional.
