Thanks, your solution works great! I wrote a sub to convert sed-ish s///n constructs to your format, making conversion from sed easier:

sub seval
{
    my ($sub) = shift;
    $vni++;
    $sub =~ s#s/([^/]+)/([^/]+)/(.+)#s/\1/\$i$vni++ == \3 ? \2 : \$&/e
+gs#;
    return $sub;
}
# Evals 's/foo/$i1++ == 1 ? bar : $&/egs'
eval(seval("s/foo/bar/2"));
[download]

Thanks again.