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.