This

The semantics are compatible with Perl's sort() function; it returns -1 if $dt1 < $dt2, 0 if $dt1 == $dt2, 1 if $dt1 > $dt2.

is not compatible with your code

say DateTime->compare( $d1, $d2 ) 
    ? $d1->ymd .' is greater than ' . $d2->ymd 
    : $d2->ymd .' is greater or equal to ' . $d1->ymd;
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Better would be e.g.

say $d1->ymd . ' is ' . ('earlier than', 'equal to', 'later than')[Dat
+eTime->compare( $d1, $d2 ) + 1] . ' ' . $d2->ymd;
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