First, since there is no seperator, I would not first look to split, but rather to m//g in list context:

my $string = "ThisIsACamelCasedString";
my @split = $string =~ /([A-Z][a-z]*)/g;
print "Parts: @split";
[download]

(This version just skips any part that does not match /[A-Z][a-z]*/; season to taste.)

Dealing with "AStringWithFTP" is trickier, at least without a dictionary. The only heuristic I may suggest is to treat sequences of upper-case characters as a word of its own if in final position or if preceding an uppercase-lowercase sequence:

my $string = "AStringWithFTPAndHTTP";
my @split = $string =~ /([A-Z](?:[A-Z]*(?=$|[A-Z][a-z])|[a-z]*))/g;
print "Parts: @split";
[download]

That this is a heuristic is clear from noting that "ACString" is split as "AC", "String", and not as "A", "C", "String". But frankly, without a dictionary, I don't think there is a real solution.

Update: Oops, pasted the wrong code. Fixed now. Also added prints to make it easier to test that it really is the code I meant to post ...

Update2: Err, pasted the wrong right code too. Sorry, salva, I did not mean to steal it. /me *blushes* (Fixed now.)