use strict;
use warnings;

my $numClients;

while ( (print "Enter Number of Clients: "), $numClients=<STDIN>,
         $numClients !~ /^\s*(?:[0]*)?[1-9]\d*\s*$/)
{
   print "Your input is NOT a positive integer!\n";
}  

$numClients += 0;  #optional conversion to numeric
print "Number of Clients: $numClients\n";

__END__
C:\PerlProjects\Monks>perl getposinteger2.pl
Enter Number of Clients:     5
Number of Clients: 5

C:\PerlProjects\Monks>perl getposinteger2.pl
Enter Number of Clients:  -23
Your input is NOT a positive integer!
Enter Number of Clients:  2 3
Your input is NOT a positive integer!
Enter Number of Clients:     14.3
Your input is NOT a positive integer!
Enter Number of Clients:   -12 23
Your input is NOT a positive integer!
Enter Number of Clients: 5
Number of Clients: 5

C:\PerlProjects\Monks>perl getposinteger2.pl
Enter Number of Clients:  00003
Number of Clients: 3