後読み否定...

gistfile1.pl

use 5.10.0;
use strict;
use warnings;
 
#perl -E 'say for q{12345 #123456 &#1234567; #12345678; &#123456789} =~ /(?<!&)#([0-9]+)/g'
for my $message ('12345', '#123456', '&#1234567;', '#12345678;', '&#12345678') {
 
    if ($message =~ m/#([0-9]+)/) {
        unless ($message =~ m/&#$1/) {
            say $1;
        }
    }
}