G'day Chuma,

I'd define your parameters and default values in a separate file. For this example, I've used JSON:

$ cat pm_11145036_parm_defaults.json
{
    "margin-left" : [ null, "margin-horizontal" ],
    "margin-right" : [ null, "margin-horizontal" ],
    "margin-top" : [ null, "margin-vertical" ],
    "margin-bottom" : [ null, "margin-vertical" ],
    "margin-horizontal" : [ null, "margin" ],
    "margin-vertical" : [ null, "margin" ],
    "margin" : [ 10, null ]
}
[download]

The following code is a rough example of how that might be used.

#!/usr/bin/env perl

use strict;
use warnings;
use autodie;

use constant {
    VALUE => 0,
    DEFER => 1,
};

use JSON;

my $json_file = 'pm_11145036_parm_defaults.json';

my $par = decode_json(
    do { local $/; open my $fh, '<', $json_file; <$fh>; }
);

sub _get {
    my ($key) = @_;

    if (! defined $par->{$key}[VALUE]) {
        _get($par->{$key}[DEFER])
    }
    else {
        return $par->{$key}[VALUE];
    }
}

sub _set {
    my ($key, $value) = @_;

    if (! defined $par->{$key}[DEFER]) {
        warn "Can't change factory default: '$key'.\n";
    }
    else {
        $par->{$key}[VALUE] = $value;
    }

    return;
}

# Some examples of usage:

print 'margin-left = ', _get('margin-left'), "\n";
_set('margin-horizontal', 20);
print 'margin-left = ', _get('margin-left'), "\n";
_set('margin-left', 30);
print 'margin-left = ', _get('margin-left'), "\n";

print 'margin = ', _get('margin'), "\n";
_set('margin', 50);
print 'margin = ', _get('margin'), "\n";
[download]

Output:

margin-left = 10
margin-left = 20
margin-left = 30
margin = 10
Can't change factory default: 'margin'.
margin = 10
[download]

Notes:

• Keeping your data separate means it can reused in all of your programs: no need to tweak %par in multiple scripts (potentially a lot of work; certainly error-prone).
• When a user has set preferences, these can be stored in a separate file for reuse across all programs; e.g. parm_USERNAME.json.
• For further reuse, put most of that code in a module; then each program just needs something like use Chuma::Layouts;.
• I only used JSON as an example. It's not intended as a recommendation: pick whatever you want.

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