As long as you're doing that ...

$ perl -e '$s="JustAnotherPerlHacker\n"; $s=~s/(?<=.)(?=[A-Z])/ /g; pr
+int $s'
Just Another Perl Hacker
[download]

By using zero-width assertions on both sides, we don't need to actually match anything. We're matching, literally, zero characters - we're matching simply a location with no length associated. Note, however, that I would make it just a bit different for a bit more flexibility:

$s=~s/(?<=\S)(?=[[:upper:]])/ /g
[download]

Match the location between a non-space and a capital letter. Using a character class gives us flexibility for other languages.

This may also be useful for splitting:

$ perl -e '$s="JustAnotherPerlHacker\n"; @a = split /(?<=\S)(?=[[:uppe
+r:]])/, $s; print "@a"'
Just Another Perl Hacker
[download]

Since split is splitting on nothing, there is nothing actually removed.