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

# General default value
my $foo = 'default';

# Let environment override the default
if (defined $ENV{FUBAR}) {
    $foo = $ENV{FUBAR};
}

# The command-line option overwrites if it exists
GetOptions("foo=s"=>\$foo);

print "The foo parameter is '$foo'\n";

##</code><code>##

$ perl foo.pl
The foo parameter is 'default'
$ export FUBAR='Gragnar'
$ perl foo.pl
The foo parameter is 'Gragnar'
$ perl foo.pl -foo=BARBAZ
The foo parameter is 'BARBAZ'
$