Very good answer, but you can simplify it to

s/(.{9}\S*\s)/$1\n/g
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Note especially the use of $1 instead of \1.

Update: inserted the \s I forgot.

Update 2:As pointed to by Roy Johnson, the \S* can be left out like this:

s/(.{9}\s)/$1\n/g
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Also, I changed {10} to {9} as the space is the tenth character.