Fixed a bug - replaced the hash value undef() with the key name.

Given @list_a and @list_b I construct an anonymous hash of @list_a and then immediately extract a slice from that hash using @list_b as the keys to fetch. @dups now contains the intersection of both lists.

@dups = @{ { map { $_, $_ } @list_a } }{ @list_b }

# Broken out.

# Duplicate each key - the key must also be the value since it will be
+ retrieved later. This creates a list useable as a hash.
map { $_, $_ } @list_a

# Create an anonymous hash from that list
{ ... }

# Fetch a slice from a hash ref using @list_b
@{ ... }{ @list_b }

# Into dups
@dups = ...
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