Here's (roughly) what Perl does when you use the -pi.bak flag on the file filename (and what you'll have to emulate):

1. Open the file "filename" for read, with handle READ
2. Rename "filename" to "filename.bak"
3. Open the file "filename" for write+truncate with handle WRITE
4. Read through all the lines from READ, modify them, then print them to WRITE