WARNING: I've never used Tk from Perl before.

FWIW, I just experimented a bit with it, and I get the error you're talking about in xclipboard. But then I tried:

my $mw = MainWindow->new();
$mw->clipboardClear();
$mw->clipboardAppend('clip');

my $selected = $mw->SelectionGet();
MainLoop;
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i.e., I followed it with SelectionGet() and MainLoop, and it printed 'clip' into the xclipboard window.

.... erm, but now I'm getting an error:

PRIMARY selection doesn't exist or form "STRING" not defined at ./tk.p
+l line 9
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Man, that's really flakey.