As with system and exec,

open (my $ARTICLE, "-|", $shell_command)
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is short for

open (my $ARTICLE, "-|", "cmd", "/x", "/c", $shell_command)
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open successfully executed cmd as requested. No error occurred, so it returned the pid of the child.

Presumably, cmd will return an error for being unable to execute ceasar. Of this you will be notified when you close the handle. It will return false with a true value for $? >> 8.