G'day arblargan,

There's probably some options you'll want to hard-code such as '--version' and '--help'. These typically run some very small part of the code and then terminate: the main application wouldn't be run. They are options without values whose operations you wouldn't want your users to change; although, you may allow some output modifications with options like '--verbose' and '--full'.

There may be some default values that you might want to hard-code or place in some sort of master configuration file. These could be overridden by user choices in a user configuration file.

You may want to allow users to set up options in an environment variable. These might be specific to a client, project, and so on: not general enough for a config file but a nuisance to restate for every run.

Then you might want command line options: items that users want to change on a run-by-run basis.

If your application is interactive, perhaps users can change options in mid-run.

I see ++Discipulus has provided some information on config files. I won't elaborate on that.

Here's some code that allows users to change starting settings with values from an environment variable; and then to override those with command line options.

#!/usr/bin/env perl -l

use strict;
use warnings;

use Getopt::Long 'GetOptionsFromString';

my ($x, $y, $z) = qw{42 fred 1.0};
my %opts = ('x=i', \$x, 'y=s', \$y, 'z=f', \$z);

print 'Option starting values:';
print "Options: x[$x] y[$y] z[$z]";

if (exists $ENV{USER_OPTS}) {
    GetOptionsFromString($ENV{USER_OPTS}, %opts);
    print 'After environment variable options processed:';
    print "Options: x[$x] y[$y] z[$z]";
}

if (@ARGV) {
    GetOptions(%opts);
    print 'After command line options processed:';
    print "Options: x[$x] y[$y] z[$z]";
}
[download]

Here's a few sample runs to show that in action.

$ pm_1196515_option_processing.pl
Option starting values:
Options: x[42] y[fred] z[1.0]

$ export USER_OPTS='-z 1.5'

$ pm_1196515_option_processing.pl
Option starting values:
Options: x[42] y[fred] z[1.0]
After environment variable options processed:
Options: x[42] y[fred] z[1.5]

$ pm_1196515_option_processing.pl -x 21
Option starting values:
Options: x[42] y[fred] z[1.0]
After environment variable options processed:
Options: x[42] y[fred] z[1.5]
After command line options processed:
Options: x[21] y[fred] z[1.5]

$ pm_1196515_option_processing.pl -y wilma -z 3.4
Option starting values:
Options: x[42] y[fred] z[1.0]
After environment variable options processed:
Options: x[42] y[fred] z[1.5]
After command line options processed:
Options: x[42] y[wilma] z[3.4]
[download]

If you want to give users the ability to change options in mid-run, here's some code I posted a few months ago to do that: "Re: How to process Getopt::Long from STDIN".