I have to admit I'd prolly have used s/\s+/ /g;, too, but it's something tr is more suited to and reputedly faster for.

tr/ / /s;

Update: I had the time, so figured 'what the heck' ...

Benchmark: running s, tr, each for at least 5 CPU seconds ...

         s:  5 wallclock secs ( 5.15 usr +  0.01 sys =  5.16 CPU) 
             @ 54047.31/s (n=278722)

        tr:  5 wallclock secs ( 5.05 usr +  0.00 sys =  5.05 CPU) 
             @ 130668.19/s (n=659613)

       Rate    s   tr
s   54047/s   -- -59%
tr 130668/s 142%   --
[download]

    --k.