This core dumps on 5.8.3 - I dunno if upgrading can be classed as a solution :-)

Update: Oh, and perlre (on 5.8.3) says:

"(?{ code })"
WARNING: This extended regular expression feature is considered highly experimental, and may be changed or deleted without notice.

There is however a simple way of making the code work: you have too many question marks. Try:

print "$_ ", /(.*)(?{$countSpaces->()})/ ? "has 1 or no spaces\n" : "h
+as more than one space\n";
[download]

this line has more than one space
 has more than one space
     that_is_5_spaces
 has more than one space
This isonespaceonly
 has more than one space
therearenospaceshere
 has 1 or no spaces
[download]

Update 2 Erm, ah. Doh. (??{ code }) is perfectly valid according to perlre too - put equally experiment. Nevermind. Using highly experimental features is probably a bad idea.