Think about it this way - you're dealing with a group of stuff. If you want to remove things from that group, you use grep. If you want to transform each thing in that group, you use map. If you want to reorder the group, you use sort.

Every single grep and map should be doing one and only one thing. You will want to chain them together, right-to-left, to make them do what you want.

@temp =
    map {
        "$_\n"
    } map {
        split
    } @file;
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Read it from right to left.

1. I'm starting with @file
2. I take each element and apply split to it. (The default is to split $_ on whitespace.)
3. I take each element of the new list and add a newline to it.
4. Put this into @temp.

Remember - you're going to have more things in your group after the first map.