The problem is that you forgot to declare '$newvar' as a local variable, so each time through the loop the result of the previous assignment is still there.

Switch it to my($newvar), or reset it each time.

Not having local variables also means that $minlength is incremented by ('+=') a random number each time. If you want to pick a number from 5 to 15, you are a little off track.

As a creative suggestion, instead of doing all that work, why not just this?

    my(@vchars) =(a..z,A..Z,0..9);

    sub rwg
    {       
          return join ('',
              map { $vchars[int(rand($#vchars+1))] }
                 5 .. 5+int(rand(10)));
    }
[download]

Change 5 to $minlength, 10 to ($maxlength-$minlength) if so desired.

Additionally, remember that rand() picks a number up to the maximum, which in this case is $#vchars, so the last letter will probably not get picked unless you +1 it.