my $pat = qr {
    (?(DEFINE)
       (?<number>    (?: [0-9]+ ))
       (?<sign>      (?: [-+] ))
       (?<expr>      (?: (?&term) (?: \s* [-+] \s* (?&expr))? ))
       (?<term>      (?: (?&factor) (?: \s* [/*] \s* (?&term))? ))
       (?<factor>    (?: (?&number) |
                         (?&sign) \s* (?&factor) |
                        \( \s* (?&expr) \s* \)))
    )
    (?&expr)
}x;
[download]

That will even parse 2*-3, and doesn't require (??{ }), or use re 'eval';.