#!/usr/bin/perl
use strict;
use warnings;

my $tail = ".D(next_out_en), .CP(n16), .CDN(n48), .Q(\n";

print "Matched: \$1 '$1', \$2: '$2', \$3: '$3'\n"
    if $tail =~ /^\s*(.+)\s+\.\s*CP\s*\(\s*(\S+)\s*\)\s*\,\s+(.+)\s*\n
+/i;
[download]

Prints:

Matched: $1 '.D(next_out_en),', $2: 'n16', $3: '.CDN(n48), .Q('
[download]

How is your code different? Maybe you don't have a newline at the end of $tail?