use warnings;
use strict;

my %fruit = (
		apple => ['red','green'],
		kiwi => 'green',
		banana => 'yellow',
	);

my %colors;
for my $f (keys %fruit) {
	for my $c ( ref $fruit{$f} ? @{ $fruit{$f} } : $fruit{$f} ) {
		push @{ $colors{$c} }, $f;
	}
}

use Data::Dump;
dd \%colors;