Updated: Just learned that the special option 'name' "<>" can be used to designate a subroutine to handle non-option arguments (see Argument callback section in the docs). So the following is not entirely correct w.r.t. a "default option"...

Getopt::Long has lots of configuration options that have their own defaults, but there's no option to define a default option and default value/behavior for it. You need to do that for yourself, but Getopt::Long can help. One of its default behaviors is to flag as errors “Options that are unknown, ambiguous or supplied with an invalid option value” (see the pass_through configuration option).

So if you do nothing special, GetOptions() will tell you when you use an unknown option on the command line.

Using cjb's sample as a start, this shows what happens when you include an unknown option on the command line:

#!/usr/bin/env perl
use 5.010;
use strict;
use warnings;

use Getopt::Long;

my $result = GetOptions (
                         h => \&help,
                         d => sub {print "\nRun DEV COMPARE\n";},
                         p => \&PROD,
                        );

say "result: $result";
bad_option()   unless $result;

say "\ncontinuing normally";
say "Doing stuff.....";
say "exiting normally";

sub PROD {print "\nRun Production COMPARE\n";}

sub help {print "\nHelp text\n";}

sub bad_option {
    print "\nInvalid option detected... Quitting!!!\n";
    exit 1;
}
[download]

And run like so (only defined options):

./pm-888730 -h -d -p

Help text

Run DEV COMPARE

Run Production COMPARE
result: 1

continuing normally
Doing stuff.....
exiting normally
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or thusly (one defined and one undefined option):

./pm-888730 -d -x

Run DEV COMPARE
Unknown option: x
result: 

Invalid option detected... Quitting!!!
[download]

Comments:

• GetOptions() returns 1 if all's well
• default is a keyword starting in Perl 5.10, so I don't use it here as a sub name (though it is possible to define a sub named default, you just need to get the calling syntax right... exercise for the reader).
• Note that the behavior here is for GetOptions() to service all valid options and also to flag the error(s). If you want to trap unknown options and quit immediately, you need to move your dispatch table outside the call to GetOptions() and trap the bad option(s) before dispatching.