use strict;
use warnings;

my $n_cookies;
while ( (print "How many cookies do you want? Enter number: "), 
        $n_cookies=<STDIN> and $n_cookies !~ /^\s*[1-9]\d*\s*$/)
{
   print "invalid input! - try again!\n";
}

$n_cookies +=0;  # trick to convert to numeric value
                 # which "deletes" leading/trailing zeroes
                 # or a substitution operation would be fine 
print "$n_cookies cookies requested!";                 

__END__
Example Run:
Note: allowing leading zero requires more complex regex
This is just a "concept example"
C:...\PerlProjects\Monks>perl commandloop.pl

How many cookies do you want? Enter number:
invalid input! - try again!
How many cookies do you want? Enter number: 0
invalid input! - try again!
How many cookies do you want? Enter number: 023
invalid input! - try again!
How many cookies do you want? Enter number:   1  23
invalid input! - try again!
How many cookies do you want? Enter number:   abc
invalid input! - try again!
How many cookies do you want? Enter number: cookies 34
invalid input! - try again!
How many cookies do you want? Enter number:    5
5 cookies requested!