Description:

FileHandle::Unget operates exactly the same as FileHandle, except that it
provides a version of ungetc that allows you to unget more than one character.
It also provides ungets to unget a string.

This module is useful if the filehandle refers to a stream for which you can't
just seek() backwards. Some operating systems support multi-byte
ungetc(), but this is not guaranteed. Use this module if you want a
portable solution. In addition, on some operating systems, eof() will not be
reset if you ungetc after having read to the end of the file.

NOTE: Using sysread() with ungetc() and other buffering functions is
still a bad idea.