You can try starting with a "templated" filename and increment it until you get past all the existing files:

my $fn = "output000";
$fn++ while -e $fn;
[download]

At that point, you'll have a filename that is just past the last one of that pattern. Minor caveat: make sure you have enough zeroes at the end of the template - or you'll start incrementing the characters. Yes, really.

ben@feynman:~$ perl -wle'$number = "one"; $number++ for 1..3624; print
+ $number'
two
[download]

:)

-- 
I hate storms, but calms undermine my spirits.
 -- Bernard Moitessier, "The Long Way"