With Tk, you probably want to look at the documentation for Tk::fileevent. One solution is to use the pipe or socketpair function to generate streams from your parent to your child and vice-versa. (perlipc gives an example of this under the heading Bidrectional Communication with Yourself.)

The idea would be is that your child could act as a server, recieving notification from the parent when an update is needed. Then it could send the information back to the parent which could know its there from Tk::fileevent. (so the parent could do it within the Tk event loop.)