... 
 my $d = shift; # $ARGV[0]
 
 if( $d =~ /(?<!\d)\d{6}(?!\d)/ ) {  print "Exact match (lookaround)\n" }
 else { print "Not Correct\n" }
 
 if( $d =~ /^\d{6}$/ ) {  print "Exact match (anchored)\n" }
 else { print "Not Correct\n" }
 ...