This is part serendipity on the part of the Tk developer and part misunderstanding on your part. That leading '+' is a unary plus and from reading perlop you'll see "Unary "+" has no effect whatsoever, even on strings.". Reading slightly above that you'll see "a string consisting of a minus sign concatenated with the identifier is returned". All this means is "+background" becomes "background" and "-background" becomes "-background".

From here it's up to the Tk developers to do something useful with the arguments. In general you just aren't supposed to pass a parameter in using both the bare and '-' prefixed form. You've just managed to highlight that Tk will take the bare parameter in preference to the '-' prefixed parameter. That's probably just the order that Tk does something with the parameters but /maybe/ that's also just related to the order that 'background' and '-background' occur inside a hash. Just do something sane and pick a style, ok? (and remember to read perlop)

Just for giggles I did a demo of your examples but in generalized form because this really has nothing to do with Tk or any other modules.

use Data::Dumper;
{ my %x = (background => 'red');
  print Dumper(\%x), "\n" }
# $VAR1 = { 'background' => 'red' };

{ my %x = (-background => 'red');
  print Dumper(\%x), "\n" }
# $VAR1 = { '-background' => 'red' };

{ my %x = (+background => 'red');
  print Dumper(\%x), "\n" }
# $VAR1 = { 'background' => 'red' };

{ my %x = (+background => 'red',
           +background => 'green');
  print Dumper(\%x), "\n" }
# $VAR1 = { 'background' => 'green' };

{ my %x = (+background => 'red',
           -background => 'green');
  print Dumper(\%x), "\n" }
# $VAR1 = { '-background' => 'green',
            'background' => 'red' };
[download]

__SIG__
use B;
printf "You are here %08x\n", unpack "L!", unpack "P4", pack
  "L!", B::svref_2object(sub{})->OUTSIDE;
[download]