If you are using X-Windows, note that the X paste buffer can only work with one contiguous string at a time, and there is no way around this. That's why selecting one string will always deselect any other string -- except for Tk widgets where "exportSelection" is false, and in those cases, the highlighted string will stay highlighted when you select something else, but you won't be able to paste it anywhere.

Well, maybe there's a way to create or locate an "alternate" paste buffer, other than the "standard" X paste buffer that is normally manipulated with mouse buttons. This would be easy within the scope of a single Perl/Tk app, but more challenging across independent apps (think "inter-process communication").

I'm not sure, but I think that on MS-Windows, you have to specifically request that a highlighted string be copied into the window-manager's paste buffer in order to make it paste-able into another window. But again, only one contiguous string can be selected at a time for this sort of copying; (and I don't even know whether or not this is part of your question).