Changes

    s'$lhs'$rhs' now does no interpolation on either side.  It used to
    interplolate $lhs but not $rhs.

    The second and third arguments of splice are now evaluated in scalar
    context (like the book says) rather than list context.

    Saying shift @foo + 20 is now a semantic error.

    The elements of argument lists for formats are now evaluated in list
    context.

    You can't do a goto into a block that is optimized away.

    It is no longer syntactically legal to use whitespace as the name
    of a variable.

    Some error messages will be different.