The best I can come up with is to use look-arounds for the spliting and then splice to concatenate single capitals. It is not perfect as you can see from the third data item.

use strict;
use warnings;

my @strings = (
   q{ThisIsACamelCasedString},
   q{AStringWithFTP},
   q{HeIsANASAAstronaut},
   );

my $rxCamel = qr
   {(?x)
       (?<=[a-z])(?=[A-Z])
       |
       (?<=[A-Z])(?=[A-Z])
   };

foreach my $string ( @strings )
{
    print qq{String: $string\n};
    my @words = split m{$rxCamel}, $string;
    for my $idx ( reverse 1 .. $#words )
    {
        if
        ( 
           $words[$idx] =~ m{^[A-Z]+$}
           and
           $words[$idx - 1] =~ m{^[A-Z]$}
        )
        {
            $words[$idx - 1] .= splice @words, $idx, 1;
        }
    }
    print qq{     $_\n} for @words;
}
[download]

Here's the output.

String: ThisIsACamelCasedString
     This
     Is
     A
     Camel
     Cased
     String
String: AStringWithFTP
     A
     String
     With
     FTP
String: HeIsANASAAstronaut
     He
     Is
     ANASA
     Astronaut
[download]

I think there will be too many corner cases for this task to be be achieved without some comparison with perhaps a dictionary list of acronyms.

I hope this is of use.

Cheers,

JohnGG