Well, your regexp won't properly identify:

1.5
-3
-2.89
.32
0.5
-.01
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...as numbers. It will also fail to reject empty strings.

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

Among other reasons:

sub is_a_num { $_[0] =~ m/^[0-9]*$/ }
my $not_at_all = '';
print is_a_num( $not_at_all ) ? "Yeppers.\n" : "Nopers.\n";
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Beware the Star of Regex!

Because if $x is an object, then your regex won't work. (Nevermind that it won't work for reals or negative numbers.)

A stringification method (which your regex assumes) isn't reliable, because I could write a stringification method to output the number in hexidecimal, Base64, or as a roman numeral (see below) but as far as Perl was concerned, it could still operate just like a number):

package RomanNumber;

use Roman 'roman';

sub new {
  my $class = shift;
  my $self  = { value => shift };
  bless $self, $class;
}

sub as_num {
  my $self = shift;
  return $self->{value};
}

sub as_string {
  my $self = shift;
  return roman($self->as_num);
}

sub compare {
  my ($a,$b) = @_;
  return ($a->as_num <=> $b->as_num);
}


use overload
  '0+'  => \&as_num,
  '""'  => \&as_string,
  '<=>' => \&compare;
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If I add a few more numeric operators (it's an incomplete example), Perl can treat it just like a number, except that it outputs a roman numeral in string context.

There are cases where I would like to require a value to be number, but I would like to give the user of a module the ability to use a 'number-like' object rather than a number.