Here is one of mine I was writing last month for me fancy index script -- i was turning it into a module. I added the callback option after coming accross this node (I said what the heck, couldn't hurt). It not only sets defaults, but does validity checks for values. Keep mind me module ain't finished, and this is from 3rd revision .. style could use some polishing, and a few more checks could be added ...

#!/usr/bin/perl -w
# the defaults that *MUST* be defined
# the default value is element 1
# each subsequent element is an acceptable value
# use quotemeta or \Q\E if you need to
# all matching is done with the global flag on,
# so anchor if you need to
# use "" for any value (or qr{^.*$}iso)

my %DEFAULTS = (
    FOLDERS_FIRST => [1,0],
    SORT_ORDER => [qw/ A D /],
    SORT_BY => [qw/ N M S/],
    ALLOW_QUERY => [1, 0], # allows ?N=A and teh like
    CACHING => [0, 1],
    EMIT_HEADER => [1, 0,
                    'text/html',
                     [ qr{ (\s? \w+ \/ \w+ \s? \;?) }iosx,
                       sub { 
                          my ($val, $matchesref, @matches) = @_;
                          carp("uh oh") if length($val) != length(join
+'',@matches);
                          $$matchesref = 1;
                          # we'll warn, but we'll still accept the val
+ue
                       } ]
                   ], 
,);

my %OPTIONALS = (
    CACHE_WHERE => [""], # here dummy
    CACHE_AS    => [""], # user here
    CACHE_SIZE  => [""], # you tell me
    KEY_SIZE    => [qr{^\d+$}], # once again, you tell me
);

use Carp;
use Data::Dumper;

print Dumper new('satin',
    SORT_BY => 'N',
    EMIT_HEADER => 'text/plain',
    CACHE_WHERE => 'on the moooon',
    KEY_SIZE => -1,);

print Dumper new('satin',
    SORT_BY => 'N',
    EMIT_HEADER => 'text/plain; charset=US-ASCII',
    KEY_SIZE => 10,);

sub new {
    my ($satin, %options)  = @_;
    my %me = map { $_ => $DEFAULTS{$_}->[0] } keys %DEFAULTS;

    for my $optionkey(keys %options) {
        my @DEFS;
        @DEFS = @{$DEFAULTS{$optionkey}} if exists $DEFAULTS{$optionke
+y};
        @DEFS = @{$OPTIONALS{$optionkey}} if exists $OPTIONALS{$option
+key};

        if(@DEFS) {
            my $optv = $options{$optionkey};
            my $matches = 0;

            for my $pattern ( @DEFS ) {
                if (ref($pattern) eq 'ARRAY') {
                    my ($pat, $sub ) = @$pattern;
                    $sub->( $optv, \$matches, $optv =~ m{$pat}g);
                }elsif( $optv =~ m{$pattern}g ) {
                    $matches++;
                    last;
                }
            }

            if( $matches ) {
                $me{$optionkey} = $optv;
            } else {
                carp "`$optv' is not a valid value for `$optionkey' --
+ please read the pod";
            }
        } else {
            carp "`$optionkey' is not a valid option -- please read th
+e pod.";
        }
    }

    return bless \%me, $satin; # i have not sinnid
}
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______crazyinsomniac_____________________________ Of all the things I've lost, I miss my mind the most. perl -e "$q=$_;map({chr unpack qq;H*;,$_}split(q;;,q*H*));print;$q/$q;"