You are mistaken.

my $commitlog = "CNR12345 commit to hook check";

my ($cnr_number) = $commitlog =~ /^(CNR[0-9]+)/m
    or die("You must enter a valid CNR\n");

print("CNR number is $cnr_number\n");
[download]

$ perl a.pl
CNR number is CNR12345
[download]