Perdido Street Station is generally considered steampunk with some cross subgenre elements thrown in for good measure. It's one of those novels that is very difficult to put an exact label on.
And since the emphasis in steampunk is on technology that is so outrageous it can only be magic--then the emphasis is on magic. There does have to be an emphasis on "clockwork" magic for it to be considered steampunk though. And was the case with League of Extraordinary Gentlemen, there are often elements of classical magic as well.
So Jules Verne and H.G. Welles wrote fantasy, then? Because at the time they wrote their novels, the technology they wrote about was so outrageous it could only be considered magic.
How about Frank Herbert's Dune? That floating fat man looks pretty magical, no? And the Spice that makes peoples eyes glow has no foundation in biology or physics. Richard Morgan's altered carbon technology is also something that to us is outrageously strange, is that magic? (and subsequently makes the books fantasy and not sci-fi?) And since we don't currently have the technology to do it, is FTL magic? In that case Battlestar Galacica is fantasy. I could go on and on.
And "Steamboy" doesn't mention magic at all. I doubt you can call that anything but steampunk.
By your reasoning, anything Sci-Fi is actually fantasy. But it's called science
fiction, which incidentally means it's
made up, and doesn't nessecarily need to be grounded in reality. But it still doesn't make it magical or fantastical (okay, it's a BIT fantastical but that itself doesn't mean it's
fantasy.)
Having to explain technology with magic is only laziness, not a proof of specific genres. Steampunk doesn't
require magic. Indeed, if you add magical properties to "steam-tech", it's no longer steampunk. It's fantasy. Adding magic to "steam-tech" removes the "tech" aspect alltogether, and makes it "Steam-magic". This is obvious, really.
In steampunk, you use
technology to explain seemingly magical properties, not the other way around. Also very very obvious.
Again. There's nothing in steampunk that is magical. It only seems that way to the technologically challenged. Magic in steampunk (or any other tech-based genre) is a crutch to people who don't know how to explain technology.
I'm not saying you have to separate the genres fantasy and steampunk. What you have to separate is the concept of steampunk and the concept of magic. Or else it's not a combination between fantasy and steampunk, it's just fantasy.
EDIT: Oops. I seem to have repeated myself something fierce here. But only because this is so obvious that it may need to be explained in different ways.
PS: I wouldn't call Perdido Street Station steampunk. It's an alien world with alien people and alien technology. Sure its industrial style setting is similar, but as far as I can remember (I read the book some 3 years ago) there's no mention of actual steam technology. Then again, my memory could possibly serve me a dish of rotten information.