Can you be specific as to how it doesn't work?

• what does your input file contain?
• what is your pattern?
• what is your replace string?
• what OS are you on?
• what output do you get (or error messages)?
• what output do you expect to get?
• what is the meaning of life?

S///g should be lowercase: s///g

use warnings; is good too.

If your pattern has special characters, you may need to escape them (quotemeta).