I guess you would want to make your regex case sensitive:

$s =~ s/([a-z])([A-Z])/$1 $2/g;
[download]

But then I think it still would not fit the description as it
woud not split something like "Test1Test2" into "Test1 Test2"
So following your line of thought it should be something like

$s =~ s/([^A-Z])([A-Z])/$1 $2/g;
[download]

si_lence