@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 = ...