##############################
# This code has been tested. #
##############################

# Step 1 - Get your starting list
my @main_list= ('Currency','USD','Asset','Equity','Country','USA');

# Step 2 - Generate the sub-list
my @sublist_indices = grep { !(($_ + 1) % 2) } (0 .. $#main_list);
my @sublist = @main_list[@sublist_indices];

# Step 3 - Do the action(s) on the sub-list
my $string = join '_', @sublist;

print "'$string'\n";