Instead of regexp recipes, you can always perform the addition and catch the exception. The FATAL keyword bellow will elevate the warning to an exception - check $0 to find if one was raised. (Normally, statements like $var = '3a' + 3 are not fatal.)

my $var = '-.3';

use warnings FATA=> 'numeric';
my $a = eval '$var + 3';
print 'Not a number' if $@;
[download]