From the docs you linked to:

> Here's how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is empty, $ARGV[0] is set to "-", which when opened gives you standard input.

So what's happening is that STDIN is re-opened.

Did you try @ARGV = ($INPUT); instead?

update

just tested, doesn't work, this will try to use $INPUT as filename not handle...

From the proposed alternatives, I like choroba's best with *ARGV = \$INPUT