The allusion is probably to Bo Derek, who was considered a perfect beauty of a woman.

For those who don't know their "history," Bo Derek is the actress who starred with Dudley Moore in the film "10". In the film, Ms. Derek, a "perfect 10", became Mr. Moore's fantasy and obsession.

http://www.climbingwashington.com/classics/boderek.htm

a stud-muffin (American, informal)
a sexually attractive and sexually active young man. She met her latest stud-muffin in the gym.
http://idioms.thefreedictionary.com/stud-muffin

a perfect ten = 10 out of 10; a full score on something

In your context I am pretty sure they mean that the guy in question he really awfully good looking, he scores 10 out of ten in the looks scale.

HTH

This is in reference to the 1980 movie "10" starring Bo Derek, considered at that time to be the benchmark for perfect female beauty.

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Note added at 11 mins (2007-01-21 16:45:36 GMT)
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I guess in this case the person says the perfect ten is a guy, but the origin of the saying remains the same.

I think you get the idea from the other answers, but in this context 'if you know what I mean' is usually a thinly camouflaged reference to sex.

*

The fluctuation of women's sizes has been erratic, size charts have changed, and now result in "vanity" sizes, at size zero and double zero.

Quoting lyrics from Andrew Lloyd Webber's 1970's EVITA piece, DON'T CRY FOR ME ARGENTINA:

"All you will see
Is a girl you once knew
Although she's dressd up to the nines
At sixes and sevens with you."

A woman fitting into a size 10, a.k.a 36/24/36 or near there, is considered "perfect" and "womanly". In those days "beauty" was a women who was curvy and regal, such as Bo Derek, who, it has been argued on some circuits, would be considered a plus-sized model today.

So, I argue that it can't be 10 out 10, since measurements, based on Bo Derek's movie 10, she's a 10 on three criteria, her bust, waist & thighs.

A woman complimenting a man being a perfect 10 works just as well, but it's based on percentages to the rest of the body, so any man of any height can have perfect measurements, but sadly, for woman today, it's more specific, and includes height.