Consider writing 2 == $val rather than $val == 2. It looks odd, but that way the common typo of = rather than == will give you an immediate compilation error rather than a nasty bug.
I often see people recommending this, but I have yet to actually see it in practice.
Is the = typo really so common that it warrants this kind of obfuscation? Not only does it "look odd," but it doesn't really make sense. You're not trying to find out if 2 equals the unknown, you want to know if the unknown equals 2--but in your code it's backwards. The truth is that I honestly can't remember the last time I made this mistake. No, I'm not bragging; my code has bugs too, and lots of them, but in my personal experience, the = typo really isn't a big problem. It's the type of mistake you make once, spend an enormous amount of time trying to debug, and upon discovering the deceptively simple cause, make sure you never make it again.
Maybe the "constant == unknown" syntax is more useful in C or C++ where you've only got one equality operator, but in Perl, you have both == and eq, and you must actually think about which one to use for each conditional, thus you're less likely to make the mistake of typing = instead of ==.
OT, but tilly, may I ask if you yourself use the "2 == $var" syntax?