What's wrong with the function? Nothing!

The only bug I see is in the code under the spoiler tag. You can't rely on a function not modifying $@, $!, $1, etc. If you need to pass these variables to a function, pass them by copy.

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.
[download]

In theory, this problem exists whenever a global is passed to a function which modifies that global. $@ is a global modified by eval.

Rethrowing with die $@ considered harmful demonstrates a similar problem.