I suggest the scientific approach...find out exactly what version breaks your implementation. If you know 5.79 works, try 5.800. Continue incremental testing until you get the undesired behavior. Then check the Changes log of libwww-perl and find out what changed for that version. That would give you at least an idea of where to look.

Other explorative options that might provide fruitful information include network sniffing (is the communication different between versions?), and maybe even throwing some debug statements into the libwww-perl code (such as in the request function) to find out where it's going astray.

Find out the when and where first...then worry about the why.