Matches one or more non-whitespace characters, followed by one or more whitespace characters, followed by 0 or more non-whitespace characters. The o indicates 'compile pattern only once'.

from perldoc perlop

 
If you want such a pattern to be compiled only once, add a "/o" after 
+the trailing delimiter.  This avoids expensive run-time recompilation
+s, and is useful when the value you are interpolating won’t change ov
+er the life of the script.
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The parens capture those non-whitspace matches into variables $1 and $2 for use within the scope.