use open qw(:std :utf8); OR 

##</code><code>##

open DEBUGFILE, '>', "debug.txt" or die $!;
binmode DEBUGFILE, ":encoding(UTF-8)";

##</code><code>##

use WWW::Mechanize;
...
# get the DOM document
#my $doc = $mech->{agent}->Document; # comment out

# get the webpage title
my $title = $mech->title;     # used instead
...
# create a utf-8 text file
open DEBUGFILE, '>', "debug.txt" or die $!;
binmode DEBUGFILE,":encoding(UTF-8)";
...