Ok, here's a bit of enhancement:

foreach $abc ("asd","","0","123","-12","1.2","-1.2","12-","-a12","a1")
{print "$abc:", is_a_num($abc) ? "Number" : "NaN", "\n";}

sub is_a_num {
   return ($_[0] =~ /^(\+|-)?([0-9]|\.)+$/) || 0 ;
}
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Thierry