That's easy enough to accommodate:

sub fullname {
    my @parts = @_;
    for ( @parts ) {
        s/(?<!\S)([A-Z])(?!\S)/$1./g;
    }
    return join " ", @parts;
}
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Of course, given that sort of regex, I guess it doesn't matter whether you join the parts before or after the substitution. (It's using negative look-behind and negative look-ahead to check that a single upper-case letter is neither preceded nor following by a non-whitespace character, and in that case, put a period after the letter.)