use strict;
use Data::Dumper;
use CGI qw(:cgi);

my $r = CGI->new();

my %hash = (
            'key1' => 'value1',
            'key2' => $r->param('doesnt_exist'),
            'key3' => 'value3',
);

foreach my $key (keys %hash) {
    print "$key: $hash{$key}\n";
}

##</code><code>##

key1: value1
key2: key3
value3: