my @color = qw(red blue black);
use Tie::IxHash;
tie my(%color), Tie::IxHash;
# Existing colors:
foreach(@color) {
    $color{$_}++;
}
# Attempt to add some:
foreach(qw(purple blue green red)) {
    $color{$_}++ or print "Ooh, that's a new one: $_\n";
}
print "I've got:\n"; 
print " * $_\n" foreach keys %color;