... seems to be completely undocumented.
Getopt::Long's documentation does document it, though
not directly (emphasis added):
The call to GetOptions() parses the command line arguments
that are present in "@ARGV" and sets the option variable
to the value "1" if the option did occur on the command
line. Otherwise, the option variable is not touched.
It's just a little leap from that
that you can take advantage of having an option's variable
set before its passed to
GetOptions,
which is what the odd code given does.
