Googling the text of your homework should easily give you the basic ideas.

Please show some effort. :)

update

You are welcome to show code examples and ask more detailed questions.

@consideration!

The OP is not empty but malformed

 <i want to know the basic idea for writing a perl script which will read a mathematical expression from user, evaluate the result and print on the screen. ></i>

should be edited not reaped.