Since you only match numbers (and, by default, whitespace between tokens is skipped), the parser will stop after matching 123.

There's no rule to "consume" the foo part, so the parser cannot reach 345.

You will have to decide what you allow between numbers, and write rules accordingly. Perhaps the simplest way is to set the skip to /\D+/. It's been a few years since I last used Parse::RecDescent, so I won't shame myself in showing the wrong syntax, but Parse::RecDescent is pretty well documented, and you should be able to find the skip syntax easily.

To be clear, the skip is a regular expression (default: /\s*/) that skips "whitespace" between tokens.