Try thinking in steps.

1. You have a list of stuff.
2. You want to get a sub-list of specific stuff.
3. You want to perform some action on that sub-list.

##############################
# 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";
[download]

Because you're dealing with every other element, some people will suggest using a hash. But, a hash has two problems:

• A hash is unordered.
• A hash only works with every other item.

The solution I (and others) have above will work for any rule for determining the sublist indices.

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We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.