It's worth noting, by the way, that using $foo != 7 as the control statement in an if statement is generally a Bad Idea. The unless statement is designed to cover that need, and using the != operator there just makes the code harder to read later when it must be maintained.
I second DrHyde (Re^3: if variants). It's definately not bad and neither is it a maintenance trouble. That whole sentence above should be removed from your text. It's just plain wrong.
The unless keyword is made to make you write code more like a natural language. It's not there to take !='s place in if expressions. They can both coexist happily and be mixed however you please.
Why the combination of unless and else is shunned by many is because it's a sort of double negation.
which just doesn't read well. Logically it's not strange but it's a funny and inappreciated way of expressing oneself.
My own (rather inconsistent) style is to use unless for exceptional behaviour, and if for expected/wanted behaviour. When using unless I usually want something, but something might stop me from doing that. With if I express that I perhaps want something, perhaps not.
print ... unless $quiet; # I want to print.
exit if $done; # I might want to exit here.
exit unless $stay; # I want to exit here, but apparently
# something is holding me back.
In English, saying "if this, do that, else do the other" makes sense and is easy for a person to parse. However "unless this, do that, else do the other" is much harder to parse because the "else" clause is a double negative (or even a triple negative if you have "unless not this"). Same applies to perl.
While you could, perhaps, argue that "if not this, do that, else do the other" is also a double negative, it's still easy to parse because it's familiar. We see it in every programming language ever.