#----------------------------------
#HERE IS WHERE I TRY TO USE BUTTONS
my $buttonTest = $mw->Button(-text => "test");

#  $t->set("3,3", $buttonTest);  #wrong

$t->windowConfigure("3,3", -window => $buttonTest); #right
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Read the perldoc for TableMatrix for embedded windows and search groups.google.com for "TableMatrix embeded window" for more examples.