How can I draw a perspective formula for ( S )-2-chlorobutane?

You can draw using different projections such as wedge-dash, sawhorse projection,fishcher,newman projection etc.,these are only for chiral carbon atoms(not necessarily optically active). This is a wedge-dash projection.In this the least atomic no. atom is put as a dash (this means that the other atoms are in front to hydrogen ) as you can imagine it is a sp3 hybridisation (tetrahedral shape 109.5 deg bet all the atoms).

Now after putting hydrogen in behind then we prioritise the atoms for the atomic no. connected to chiral carbon, and if the atomic nos. are same then we go to the adjacent atoms and compare the atomic nos. again (we can do it by numbering the atoms connected to chiral carbon acc to its atomic no.).

Then arrange the atoms in such away that the numbering consecutively goes anti-clockwise.in this case the Cl is highest atomic no. atom so it is no. 1,c2h5 is no.2and ch3 is no. 3. and in the diag it goes in anticlockwise direction.here s(sinister) is leftward and r(rectus) is rightward.