package Foo::Bar::Quux;

print "now in runtime...\n";

BEGIN { 
    (my $package = __PACKAGE__) =~ s!::!/!g;
    my $file     = $INC{$package.'.pm'};
    open DATA , '<', $file
        or die "Can't open '$file': $!\n";

    my $data = 0;
    while (<DATA>) { 
        /^__DATA__$/ and $data = 1 and next;
        next unless $data;
        print "DATA: $_";
    };
}
1;
__DATA__
foo
bar
quux
now in runtime...
now in main