This will make @myarray = undef.

I suspect this is just a case of casual, imprecise language, but you are likely to mislead people with that particular phrasing:

    my @myarray = undef;
    print "\@myarray contains ", 0+@myarray, " elements.\n";
__END__
Produces:
    @myarray contains 1 elements.
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Because     @myarray = undef; is the same as     @myarray = ( undef ); which isn't at all the same as     undef @myarray; undef is a function that undefines what is passed to it and then returns a not-defined() scalar value.

                - tye