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$c1 = [ $r, $g, $b ];
$c2 = contrasting_colour( @c1 );
...
                         255 - $c->[2] );
   return sprintf "#%02X%02X%02X",  $r, $g, $b;
}

sub contrasting_colour {
   my ( $c ) = @_;
...
                         ($c->[2] + 128) % 256,  );
   return [ $r, $g, $b ];
}