See the docs for Date::Calc:

o "$Dd = Delta_Days($year1,$month1,$day1, $year2,$month2,$day2);"

This function returns the difference in days between the two given dates.

The result is positive if the two dates are in chronological order, i.e., if date #1 comes chronologically BEFORE date #2, and negative if the order of the two dates is reversed.

The result is zero if the two dates are identical.

So the '> 30' part, combined w/the ternary operator is saying: "if ($3, $months{$1}, $2) is more than 30 days before ($today2, $today1, $today[0]) then put $line into @log_lines, otherwise keep_line($line)"