From open:

As a special case the 3 arg form with a read/write mode and the third argument being undef:

 open(TMP, "+>", undef) or die ...

opens a filehandle to an anonymous temporary file. Also using "+<" works for symmetry, but you really should consider writing something to the temporary file first. You will need to seek() to do the reading.

perl -le 'print defined($ARGV[0])?"defined":"undefined"'
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prints "undefined"

Update: Sorry, posted this just as blazar updated Re: 3-arg open() does not give warnings!? with the right quote! =D