print foo( 3 ), "\n";     # 3

$@ = 3;
print foo( $@ ), "\n";    # 0  Bad. Passing special var.

$@ = 3;
print foo( "$@" ), "\n";  # 3  There we go.

$@ = 3;
my $e = $@;
print foo( $e ), "\n";    # 3
print $e, "\n";           # 3  Still avilable to us.