# Do stuff if $list_element is in @a_list
if ($list_element =~ @a_list) {
  ... do stuff ...
}

# ... OR ...

# Loop through a bunch of list elements
foreach @list_elements {

  # Do stuff if the current element is in @a_list
  when @a_list      { ... do stuff ... }
  
  # If not in @a_list, what about @other_list ?
  when @other_list  { ... do stuff ... }  
}