triple-click selection with Perl selection code

[Adam Spiers]

The urxvt(1) man page says:

    -tcw
        Change the meaning of triple-click selection with the left mouse button.
        Only effective when the original (non-perl) selection code is in-use.
        Instead of selecting a full line it will extend the selection to the end of
        the logical line only. resource tripleclickwords.

However it's not clear how triple-click behaviour is determined when
the Perl selection code *is* in use.  I can't see anything in the
'selection' extension relating to it, for example.  How can I stop
triple clicks selecting the full line when I'm using the Perl
selection extension (which after all is the default) ?

Thanks!