Even though perl allows you to enter print statements quickly, there are plenty of situations where a debugger will help. One example is

Say after a long processing (e.g. after reading 1000 lines), you notice that something is wrong (e.g. a variable has invalid data).

Wouldn't it be nice if I can stop at that point and inspect all the variables? I do not need a GUI debugger, but I find debuggers to be very useful (-d option in perl).

If debuggers allow conditional break points, you can even break after reading every 10 lines (or when a variable has value X).