Well, maybe you should learn how to :map your <PageDown> key to the thing your terminal is sending? I sometimes stumbled upon that, too - but that can easily be resolved by LARTing the sysadmin that is responsible for that broken termcap entry. Don't blame vi.

For you said it was a Debian box - the default vi is nvi on that machines. And don't blame vi your ignorance (sorry to call it that way - but it simply is nothing else) for the standard vi behavior that causes you to get out of insert mode when hitting a cursor key. apt-get install vim and :set esckeys. :help esckeys to see what it does.
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use signature; signature(" So long\nAlfie");