Hi,

chomp $d;
if($d =~ m/^\d{6}$/) { ### use ^ and $ for start and end
 print "Exact match\n";
}
else {
print "Not Correct\n";
}
[download]

should do the trick.
8 digits will contain (a.k.a. match) 6 digits and therefore succeed.
update: chomp your input to remove the trailing newline.
Regards,
svenXY