It isn't so much that the -keep option is a problem, more that no value was given for the -keep option. From Regexp::Common::number:

Under -keep (see Regexp::Common):

$1

captures the entire number

$2

captures the optional sign of the number

$3

captures the complete set of digits

It is pretty sad (IMO) that RE_num_real() appears to not complain about getting options that it does not support ("3", ",", and "2"), nor about getting a value for -keep that it does not support ("-group"), nor about it getting an odd number of arguments when it is expecting pairs of arguments.

It should simply die when any of those things happen.