You said:

sub _is_start_pattern {
 my ($class, $line) = @_;
 return ($line =~ $class->START_PATTERN);
 # I really wish we could do
 #return ($line =~ $class::START_PATTERN);
}
[download]

Well, if you really want to you can always do something like this:

{
    package PB_File;
    
    our $START_PATTERN = "base-pattern";
    
    sub pattern {
        my $class = shift;
        {
            no strict 'refs';
            ${"${class}::START_PATTERN"};
        };
    }
}

{
    package An_AR;
    use base qw(PB_File);
    
    our $START_PATTERN = "sub-pattern";
};

use Test::More tests => 2;
is( PB_File->pattern, 'base-pattern' );
is( An_AR->pattern, 'sub-pattern' );
[download]

However, for me, you're not repeating any more information by having a method call. Compare:

our $START_PATTERN = qr(^(\d)\s+\1$);
sub START_PATTERN { qr(^(\d)\s+\1$) };
[download]

Okay - you have to type one more character :-)

You also have the advantage that, if you use methods, any future code maintainer doesn't have to learn (or remember) that some package globals are "special".