I don't know a tutorial, just the man-pages.

But here comes a short explanation:

printf take a pattern string and a list of value, to fill into the pattern.
So the basic form is:

printf "pattern", value, value, value;

This pattern can hold any kind of text, including newlines, if you put it between doubles.
The pattern contains a placeholder for each value in the list.

The placeholders look like %n.mt, where the % is the begin of the placeholder, n is the length, m is the decimals if you have a number and t is the type of value.

n and m are optional, t can be one of a long list of which are s,d,f some examples.
s is for strings, d is for integers and f is for floats.
The complete list and further info can be found here

Transforming your example to this gives:

printf "%18s %18s %18s\n", $ip_date_time[$i]{ip}, $ip_date_time[$i]{date}, $ip_date_time[$i]{time};

update: added link to perlfunc:sprintf

update 2:there is a tutorial in the tutorial section (where else :-): Using (s)printf()