You're regex appears to be inverted if you want to change .txt to .html. Also, a global modifier is not necessary.

$newName =~ s/html$/txt/g;
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The following would probably get you what you want:

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;
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