In this particular case, I prefer to use split as demonstrated by EvanK. However, if you insisted on regex, specially for irregular patterns (e.g. not always ".declare"), than I would use simpler regex:

$ perl -wle '$_=shift; /\S+\s+(\S+)/ and print $1' ".declare cube_no  
+      9999999.999"
cube_no
[download]

But still, on this "regular irregular patterns", splitting on spaces (or other possible delimiters) still wins over regex in my toolbag.

$ perl -wle '$_=shift; print +(split)[1]' ".declare cube_no        999
+9999.999"
cube_no
[download]