...
 my @nums = 1 .. 30;
 my $reg = qr{^(\d+)$ (??{ $1 % 3 ? '(?!)' : '' })}x;
 print join ',', grep /$reg/, @nums;
 ...

##</code><code>##

 ...
 my $reg = qr{ ^((?>\d+)) (??{ !!($1 % 3) and '(?!)' }) }x;
 ...