i'd do something like:

#!/usr/bin/perl -w
use strict;

use Getopt::Long;


my( $username, $password, $group );
my %params = (
    'u=s' => \$username,
    'p=s' => \$password,
    'g=s' => \$group,
);

sub usage
{
    die << "USAGE";
Usage: 
  $0 -u username -p password -g group [-others]

Description:
  whatever...

USAGE
}

usage() unless( GetOptions( %params ) );
for( values %params )
{
    usage() unless defined ${ $_ };
}
[download]

• this captures the return code from GetOptions (please do this!)
• i tend not to customize usage information (one doc for everyone, and they should read it!)
• there's no $err_msg variable in file scope for the rest of the program (remember to use undef when you're done with it)

~Particle *accelerates*