What are you trying to accomplish here? Have you read man page of sed ?

     The sed utility is a stream editor that reads  one  or  more
     text  files,  makes editing changes according to a script of
     editing commands, and writes the results to standard output.
[download]

Since you are already using perl, why not just use perl to perform substitution (a lot of monks had already given out code) instead of *nix sed, then your code will be portable (across other OSes, like windows).