$_ = 'A A Jones'; s[(?<=[A-Z])(?=\s)][.]g; print;;
A. A. Jones

$_ = 'Bob J Smith'; s[(?<=[A-Z])(?=\s)][.]g; print;;
Bob J. Smith

$_ = 'Dr P J van Houten'; s[(?<=[A-Z])(?=\s)][.]g; print;;
Dr P. J. van Houten