So you should now have all almost the pieces you need to work out how to manage not changing button color in chg_btn when it ought not be changed. The missing part is passing the current sensor state into chg_btn in some fashion. Where you set up button call backs you can:

my %context;

...
$button->bind('<ButtonRelease-1>' => sub{chg_btn(\%context, @_});
[download]

and something similar wherever you set up the sensor handling callback/sub. You can then

sub sensorHandler {
   my ($context, ...) @_;
   
   $context->{sensorOn} = $data !~ /(?<![\w-])OFF(?![\w-])/i;
   ...
}
[download]

and in your button handler you can

sub chg_btn { 
   my ($context, ...) = @_;

   $btn1->configure(-background => "green", -foreground => "black") 
      if !$context->{sensorOn};
}
[download]