Good spot!

I think the ellipsis "keyword" can be best explained as being equivalent to

die "Unimplemented";
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Edit: And the documentation says at the end of the first paragraph:

Perl throws an exception with the text Unimplemented

For me was unclear what does mean "throws an exception". Kinda vegetable I am unfamiliar with.

Refer to perl's glossary (perlglossary) for 'exception' and 'exception handling'.
Bill

There are two uses for '...' - as:

1. part of an expression - see Range Operators
2. statement on its own - equivalent to die "Unimplemented"; - see The Ellipsis Statement

And so...

I don't understand why '...; print 1;' exits after ellipsis and not proceeds to print (v5.28). Can someone explain?

The sequence ...; print 1; does not proceed to print, because it dies before the print, just at the ...;

I understand why 'print ...' breaks: because it makes syntax error.

Yes. Because it is not a statement on its own, it is interpreted as the infix range operator, which lacks arguments - so "syntax error".

I sometimes use an ellipsis statement in example code to indicate "stuff goes here" and "this is not executable". In the example that branch of the code is not expected to be executed.

I sometimes use an ellipsis in code I am writing to say "work needed here" and to give me a sharp whack upside of the head if that branch of the code does get executed. In a test environment exception handling can detect unimplemented code that way and report it so appropriate tests can be written that will fail with a useful message until the code is implemented.

If it's just a no-op, try triple semi-colons as visual indicator

;;;

or a comment

#TODO yadda yadda

If you want a warning instead of a die you could define your own sub __todo__

use strict;
use warnings;
use Data::Dump qw/pp dd/;

package BLA;

# poor mans import
BEGIN {
    *__todo__ = \&TODO::__todo__;
}



sub add {
    my ( $arg1,$arg2 ) = @_ ;

    __todo__ "check argument type";

    return $arg1+$arg2;
}

add 1,2;


# other module
package TODO;
use Carp;

sub __todo__ {

    carp "TODO: $_[0]";
}
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TODO: check argument type at d:/tmp/pm/t_carp.pl line 17.
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update

improved demo with packages