use Getopt::Std;
my %opts;

getopts( 'dhc:', \%opts );      # usw, -c for cfg-file
$opts{'c'} and $opts{'c'} !~ m{^/([\w\.]+/)*[\w\.]+$} and &usage;

my $cfg = $opts{'c'} ? $opts{'c'} : '/default/path/to.conf';

open CFG, '<'.$cfg or die;
while(<CFG>) {
   # do sth like ...
   /^\s*(.*?)\s*: (.*)$/;  
   $cfg{ $1 } = $2;
}
foreach (keys %cfg) {
   $opts{$_} and $cfg{$_} = $opts{$_};
}