I don't know a thing about Gtk2, but the example in the gtk2 perldoc has a good fit for your lines, provided your $button_poll and $button_exit are simply button objects.

my $button = Gtk2::Button->new ('Quit');
  $button->signal_connect (clicked => sub { Gtk2->main_quit });
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So your first line should probably look like this:

# old line Gtk2::GSignal->connect($button_poll,"clicked", \&poll_host)
+;
$button_poll->signal_connect (clicked => \&poll_host);
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