With some prodding from shmem, I was able to make progress, culminating in the code post above for "Update 2".

However, I still can't match the text I want, becuase the page fetched with WWW::Mech differs from the text I get when I open the url in firefox and do view source.

This is either no longer a javascript issue, or if it is a javascript issue I don't understand how to troubleshoot it.

At any rate, to simply reiterate the test case for my problem (basically the same as Update 2 above, but shorter and simpler):

use strict;
use warnings;

use WWW::Mechanize;
use Data::Dumper;

my $price = '249,9';
print "price: $price\n";

my $url = 'http://www.baur.de/is-bin/INTERSHOP.enfinity/WFS/Baur-BaurD
+e-Site/de_DE/-/EUR/BV_ExternalCall-Start?ArticleNo=515358&NUMSArt=444
+3504&NUMSArtPc=4488615&AffiliateID=pangora-bd&Name=pangora-produktdat
+en-baur&ActionID=preis-produkt-suche-baur&WKZ=79&IWL=101';

my $mech = WWW::Mechanize->new();
$mech->agent('Firefox/1.0 (Windows; U; Win98; en-US; Localization; rv:
+1.4) Gecko/20030624 Netscape/7.1 (ax)');
$mech->get( $url );
my $html = $mech->content;
print "html from $url doesn't match $price\n" unless $html =~ /$price/
+ ;
print "but paste into browser and view source, and it does\n";
print "final url after firefox redirect (but not www::mech redirect) i
+s something like " . 'http://www.baur.de/is-bin/INTERSHOP.enfinity/WF
+S/Baur-BaurDe-Site/de_DE/-/EUR/BV_DisplayProductInformation-ArticleNo
+;sid=7oVhaTsE5oZsaX6rnON4q25Uv6S6Ixu_PzIwW50ajEGxS04TwoV1a_bGFYiItw==
+?ArticleNo=515358&ls=0&firstPage=true&showGewinnspiel=true&showW3B=fa
+lse' . "\n";
# uncomment this to print html, which is totally different from what y
+ou get from firefox, show source.
# print "html: $html";
[download]

Can anyone prod me further in the right direction?

PS: I'm still wondering if I would get better results with Win32::IE::Mechanize or WWW::Selenium or Mozilla::Mechanize.

UPDATE: better user agent string after perrin comment.