I can't see how a script could be invoked if you click on a link on one of the legacy pages. Otherwise, perl can do about the same as PHP, but with a different syntax.
The obvious way to go is frames. What's more important to them; function or no frames? Popups would be the second choice in my opinion.
Maybe if you design the first window to be some sort of floating menu without all the icons, menus etc. and have a big popup window (main window) for the content...? Would they tolerate that?
(Well, now that I think about it, you could do it in perl. Without changing code on the legacy pages. If you invoke the script to read the old page, find all the links in it and then write the page to the browser substituting all links for links to your script. That could work. The legacy pages would be unchanged but you would change them on the fly as you write them to the browser. Would that be acceptable?
eg. any link in your menu points to your script:
<a href="
script.pl pagename.htm">Linktext</a>
where
pagename.htm is the page you're after
Your script reads which page the browser wants (pagename.htm)
reads the file and substituts all links
<a href="page_2.htm>Linktext</a> for
<a href="
script.pl page_2.htm>Linktext</a>
The script also writes all the menus etc.
Could work..., Might not be that hard even.
Try asking in the perl section about this (as I'm not really that good at perl)
Good luck
