use strict;
use warnings;

my $file = "config.txt";
my $default_dir = "";
my @config;
my $content;

if (-f $file) {
    open CONF, "<$file"; 
    $content = <CONF>;
    close CONF;

    if ($content =~ /DEFAULT_DIR/) {
        @config = split /:/,$content;
        $default_dir = $config[1];
    }
}


my $new_default = "default_path";

if($new_default ne $default_dir) {
    open CONF, ">$file";
    print CONF "DEFAULT_DIR:".$new_default;
    close CONF;
}