I wouldn't use Javascript for getting at the text:

print $mech->text
[download]

Alternatively, if you're really interested in the textContent of a specific element, use

print $element->{textContent}
[download]

Thanks to MozRepl::RemoteObject, almost all stuff you can get at by Javascript, you can get at by using Perl.

#!perl -w
use strict;
use WWW::Mechanize::Firefox::DSL;

get 'http://perlmonks.org';
print text
[download]