Since I don't know anything about the "Opsware::NAS::Client" module or the "$nas" or "$res" objects you're using, I have to guess that the $res->result() thing returns a list of lines. If that's true, then you probably want the "grep" function rather than a "foreach" loop:

print grep /^SystemImage: /, $res->result();  # replaces your "foreach
+" block
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If there are two or more "SystemImage" lines in the list being returned, you'll see all of them printed with that approach.