Second question.

Why was that the case?

Why did you have to put the parentheses in? Or why did that make you sad?

If it made you sad because you felt there should be a way of writing it without parentheses, then your instincts are sound.

The trouble, as theDamian explains in Perl Best Practices, "Mapping and Grepping", is that

when the first argument to a map or grep is specified as an expression, it becomes harder to distinguish from the remaining arguments.

His recommendation is to say

map BLOCK LIST

which in your case would be

my @small_files = grep { -s < 10000 } @ARGV;