Welcome to the wonderful world of operator precedence.

Instead of simply parsing operators from right to left or left to right, Perl (and most other programming languages) uses a scheme where certain operators (such as '<') are parsed before certain others (such as the '?:' conditional operator). See 'perldoc perlop' for the entire list of operators and their precedence.

That's why

print 3 + 2 * 5;
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In your case, since '<' has a higher precedence than '?:', the '$lines > 1' statement is parsed first without the help of brackets.

That's not to say that you don't need the brackets there. Perl doesn't need them, but frequently we programmers need them to keep track of what's going on.

stephen