$newName =~ s/html$/txt/g;   

##</code><code>##

use strict;
use warnings;

my $directory = 'C:\Perl';

chdir($directory) or die "Can't chdir to $directory $!"; 

opendir my $dh, $directory or die "Couldn't opendir: $!\n";
foreach (readdir $dh) {
	next if /^\.+$/;
	if ((my $html = $_) =~ s/txt$/html/) {
		rename($_, $html) or die "Can't rename: $!";
	}
}
closedir $dh;