From open:

You can put a '+' in front of the '>' or '<' to indicate that you want both read and write access to the file; thus '+<' is almost always preferred for read/write updates--the '+>' mode would clobber the file first. You can't usually use either read-write mode for updating textfiles, since they have variable length records.

I'm assuming INP is a text file so perhaps just '<' is a better file mode.

update: added link to perlfunc and suggest you follow advice of toolic and Fletch