Operator Precedence, again.

A ? B = C : D = E
# resolves
( A ? ( B = C ) : D ) = E
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So, no matter what A evaluates to, E is being assigned, either to "B = C" (which is the same as assigning to B) or D. In your case B and D are identical, so your ?: operator doesn't do anything useful. Solution:

@_ ? ( $param2 = shift ) : ( $param2 = "Shift" );
# and while we're at it, of course, refactor that
$param2 = @_ ? shift : "Shift"
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The last line demonstrates why the precedence of "?:" vs. "=" is actually sensible. HTH