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$ perl -le '$_="1 2 3 4 5 6"; s/^((?:[^\s]*\s){3}[^\s]*)\s/$1\n/ and p
+rint'
1 2 3 4
5 6

s/^([^\s]*(?:\s[^\s]*){3})\s/$1\n/

s/
    ^             # Start at the beginning.
...
     )            # Stop capturing.
    \s            # Match another space.
/$1\n/x           # Replace with what we captured and a newline.