With

use strict;

my %hash = map
{
      (split /:/, $_ )[0] => (split /:/, $_ )[2]
}
<DATA>;

print join ",", sort keys %hash; #key1,key2

__DATA__
key1:useless:value1
key2:useless:value2
[download]

I get :

key1,key2
[download]

The thing is, I do the same thing as your code in mine, two times : the first time, with a first array given to the map function, it works fine ; but the second time, with another array, it "inverts" the keys and the values.