Technically speaking, this is so because the "if" in the first example is a statement modifier, while the "if" in the second example is a compound statement (see: The Camel, 3rd Version, page 112-115, or perldoc perlsyn), and compound statement syntax requires the parens, while statement modifier syntax does not, but takes an expression without parens.

Psychologically speaking (*why* is this so), I have no idea.

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com