Another variation on Shendal's solution could be to allow for decimals. I hardly ever work with whole numbers (not through choice, but because of the data we are responsible for here).

my $number =  <STDIN>;
chomp $number;

if ($number !~ /^\d+\.?\d*$/) {
    print "$number is not a number.\n";
    exit 1;
} else {
    print "$number is a number!\n";
}
[download]

I hope this proves useful to someone. mrmick