What you need to do is create a subprocess. If this were C/C++, you would create a thread, but I don't think you can do that with Perl/TK on Windows.

Anyway, the subprocess does all the time-consuming stuff and sends back progress/error/completion reports via an IPC mechanism (e.g. a socket). You can use Tk::Event->fileevent for the parent process to get these notifications. Meanwhile the parent process is responding to normal window messages.

Update: Well, I spoke too soon. fork() blows chunks with Perk/Tk under Win32 and fileevent doesn't work either (althought it may "soon"). You can create a subprocess using Win32::Process. For one approach to IPC, see this.