Your code mixes up "$it" and "$item". You had the right idea in the "foreach my $item (in $Contacts->{Items}){" - you should use $item inside that loop, and delete the $it->GetFirst/GetNext stuff.

Anyway, once you get your contact item loaded into $item, you can :

   $item->{Categories} = 'Blue Category, This is another, This is one 
+category, and green category too';
   # Note the syntax here requires curlies, to SET the property
   # You can omit the curlies when GETTING the properties, which I thi
+nk is safer. 
   #Eg: $Contacts->Items  , instead of $Contacts->{Items}

   $item->Save; # This is the step you were looking for !
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