Last, but not least, let's say that you store your regular expressions in a config file or something. In many such situations, you can't change the application's logic to say "don't match this pattern." Yet, there's still a way, with negative lookahead assertions.

my $re = qr/(?!\A.*38[0-9]{8}[A-Z]{3}.*\z)/;
if ($reference =~ m/$re/)
{
    # 38nnnnnnnnXXX pattern was not found
    $reference = "";
}
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