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$ perl -le '$_ = q{Dr P J van Houten MD};
> s[(?<=[A-Z])(?=\s)][.]g;
...
> print;'
Dr P. J. van Houten MD. FRCS
$

$ perl -le '$_ = q{Dr P J van Houten MD FRCS};
> s{(?:(?<=\A[A-Z])|(?<=\s[A-Z]))(?=\s)}{.}g;
> print;'
Dr P. J. van Houten MD FRCS
$