Here is one possible implementation:

use strict;
use warnings;
use Getopt::Long;


my $demo = 0;   # default value (false)
my $test = 0;   # default value (false)
GetOptions ('demo' => \$demo, 'test' => \$test);

###  Once command line is parsed using GetOpts, THEN execute code 
###    based upon the flags which are set.
###  GetOpts removes what it knows about from @ARGV - if there is
###    anything left, then you have a syntax error.
###  It is up to you to check for contradictory options, like
###    perhaps asking for --demo and --test at the same time!

if (@ARGV)
{
   print "Illegal syntax \'@ARGV\' is bogus!\n";
   printHelp();
}

if (!($demo or $test))
{
   print "No option specified!\n";
   printHelp();
}   

if (!($demo xor $test)) #test for inconsistent options
{
   print "Only one option allowed!\n";
   printHelp();
}  

sub printHelp
{
  print "*** print something useful for help message**\n";
  exit (1);  #error exit to shell - the command "didn't work"
}  

### actual "code" is here
###

demo() if $demo;  #simple for this scenario
test() if $test;

sub demo
{
   print "doing a demo\n";
}

sub test
{
   print "doing a test\n";
}

__END__
example runs:

>perl longopts.pl
No option specified!
*** print something useful for help message**

>perl longopts.pl -d abc
Illegal syntax 'abc' is bogus!
*** print something useful for help message**

>perl longopts.pl -d -t
Only one option allowed!
*** print something useful for help message**

>perl longopts.pl adf
Illegal syntax 'adf' is bogus!
*** print something useful for help message**

>perl longopts.pl -d
doing a demo

>perl longopts.pl -t
doing a test
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