I don't know how good IO::File's docs are, but check these docs (to which I already linked):

The values for WHENCE [which you called OFFSET] are 0 to set the new position in bytes to POSITION, 1 to set it to the current position plus POSITION, and 2 to set it to EOF plus POSITION (typically negative). For WHENCE you may use the constants SEEK_SET , SEEK_CUR , and SEEK_END (start of the file, current position, end of the file) from the Fcntl module. Returns 1 upon success, 0 otherwise.

In other words, the second argument determines whether the first argument is relative to the front of the file, to the current position in the file, or to the end of the file.