If this were for in-memory handling, instead of in a file, I'd think of Tie::IxHash. That implements a "hash" that still acts like a hash in all respects, thus for example, each key must be unique; but in addition, it preserves the order of addition, when you list its keys/values. Here, I only use the keys.

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;
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Result:

Ooh, that's a new one: purple
Ooh, that's a new one: green
I've got:
 * red
 * blue
 * black
 * purple
 * green

As you can see, it's easy to spot new additions, dupes are prevented, plus the original order of addition is preserved.

If this is for a file, you can just append the new color addition to the end of the file, when you encounter a new one. You then don't even need to use Tie::IxHash — a plain hash will do.