Um, dunno, how about these questions.

Is ssh in the path of the user the web server runs as (webrun, nobody.nogroup, etc)? Try putting it in /bin..

Is the web server user registered in .ssh?

What is your ssh site config file like?

Maybe your configs do not allow some users to do things they need to do like ssh, icmp, etc?

Are you sure it is hanging, or that it worked not once? (ssh "needs" to hang a little on close, see openssh.org). Sounds like this isn't the problem though..

Does the Net::SSH wrapper for the binary also fail??