See perldoc -f join.

Update:

On second read of your original question I noticed that your code snippet may not have been illustrating what you actually intended to do. Maybe you wanted to retain the strings as separate scalar units:

( $this, $that, $the, $other ) = @ARGV;
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But if that's what you need, what's wrong with keeping them in the original array, or assigning them to a new array?