my %context;

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

##</code><code>##

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

##</code><code>##

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

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