What you are looking for is getting camera perspective right? Well you can just use the scaleX and scaleY properties as Sam suggested but if you really want to create a true perspective view, would be better write down some maths.
For this trick you will have to use a classic theorem in mathematics called Thales's Theorem, have you ever heard of it? Well it is very simple, and useful for this case. To represent the triangle, you can check the following diagram I made.
First let's indtroduce what the variables are:
* Focal length: the respective relative distance between the camera screen to the human eye.
* Z position: the distance which the figure (sprite) is far from the screen camera.
* H and h: sprite and screen height respectively.
Let fl be the focal length, z be the z position, H and h the heights we have:
H / h = (fl + z) / fl
Since we are trying to discover the scale, h / H is our scale ratio, so basically we can define the scale variable as:
s = fl / (fl + z)
Now we can define these variables in our code. Since z is the respective distance between the camera screen to the sprite we have, 0 would be the minimum value, this means that our object would be exactly in front of the camera and for example 1000 would be a very long distance between the camera and the object.
I would recommend 300 - 500 of focal length to get a nice perspective. Having z as a value 0 <= z <= 1000 for example, the focal length would be terrible if less than 50 because 40 / (40 + 1000) results in a ratio of aproximately 0.038 and as the sprite approaches the camera, having a z position of 500 would result in 0.088 which still very low.
Writing down everything we defined:
function scaleObject(obj:MovieClip, z:Number, focalLength:Number):Void
var scale:Number = focalLength / (focalLength + z);
obj._xscale = obj._yscale = scale;
// note: Long time I don't work with AS2 so depending of the scale you get
// you can multiply it by 100.
If you would like to movement your object, just define a speed variable, for example zspeed = 10 and to approach the sprite to the camera subtract z from zspeed or if you want to zoom out, just sum z to zspeed.
var z:Number = 500;
var focalLength:Number = 400;
var zspeed:Number = 10;
scaleObject(sprite_mc, z, focalLength);
z -= zspeed;
Hope that helps.