I am writing the script for "option" while getting arguments from command line, and below I have code,

use strict;
use warnings;
use Getopt::Std;

my %options;
if (scalar @ARGV < 3) {usage()};

sub usage
{
    print STDERR << "EOF";
    \nThis program does...
    usage: $0 [-hcfo]

            -h  : this (help) message
            -c  : config file
            -o  : Fj xcf release file
            -x  : output file

EOF
exit;
}

getopts ("hc:f:o:", \%options);
usage() if $options{h};

my $source_file= $options{o}; 
my $dest_file= $options{x};
my $config_file= $options{c};
[download]

And in above code, I want to "-o" to be optional for example if user don't provide "-o(filename)" that means value for $source_file then I want to take default file suppose "source.txt"; Could any one guide me how can that be done?Please. Cheers