Absolutely right! Using $_ in the sub is bad and "15not" += 0; becomes 15. At the time I was just trying to understand the advice given in Far More Than Everything You've Ever Wanted to Know About... I must confess that I don't get it. Applying the advice given doesn't seem to work in all cases and so of not much use. Using Regex::Common is far and away the best solution that I have found.

New version of the code that still doesn't work.

#!/usr/bin/perl -w
use strict;

sub i_am_a_number {
    my $val = shift;
    no warnings; # += 0 on a string sets off a warning

    # make sure that zero is counted as a number
    # ($_ += 0) alone fails on zero.
    if ($val =~ /^\d+$/ || ($val += 0)) {
        return 1;
    }
    return 0;
}

#############################################################
# take the sub on a test spin

my @list = (
    # Zeros
    0.00, .00, 0. , 0, -0, 00,
    # Integers
    035, 35, +19, "045 ", " 045", -2,
    #Floats
    .15, 5.5, -34.530,
    # Not numbers
    " 0  1 ", " 0  0 ", " 24  0 ", " 15hi ", "2isnotanumber", "hi!"
);
for (@list) {
    if (i_am_a_number($_) || i_am_a_number($_)) {
        print "$_ is a number!\n";
    } else {
        print "$_ is not a number!\n";
    }
}
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()-()
 \"/
  `