print "Content-type: text/plain\n\n";
print "Hi";

When this script is executed under Apache::Registry, it becomes wrapped in a handler( )subroutine, like this:

package Apache::ROOT::perl::test_2epl;
use Apache qw(exit);
sub handler {
    print "Content-type: text/plain\n\n";
    print "Hi";
}

If we happen to put an __END__ tag in the code, like this:

print "Content-type: text/plain\n\n";
print "Hi";
__END__
Some text that wouldn't be normally executed

it will be turned into:

package Apache::ROOT::perl::test_2epl;
use Apache qw(exit);
sub handler {
    print "Content-type: text/plain\n\n";
    print "Hi";
    __END__
    Some text that wouldn't be normally executed
}

When issuing a request to /perl/test.pl, the following error will then be reported:

Missing right bracket at .... line 4, at end of line

Perl cuts everything after the __END__ tag. Therefore, the subroutine handler( )'s closing curly bracket is not seen by Perl. The same applies to the __DATA__ tag.