#! perl
use strict;
use warnings;

my $len = 7;

for my $s ('    999', '   9999', '9999999', '      9', '  99  9', '   9')
{
    if (length $s == $len && $s =~ /^\s*\d+$/)
    {
        printf "%-*s matches\n",        $len + 2, "|$s|";
    }
    else
    {
        printf "%-*s does not match\n", $len + 2, "|$s|";
    }
}