Id do something like this myself. Except id probably not use look ahead and instead would approach it a different way.

I was annoyed with the lookahead myself, but it's unlikely to be a big deal, and I could not at the time see any way to avoid it. After some thinking, however, I believe I see a way to avoid looking ahead more than once -- just include it in the first alternation, which is matched precisely once on a successful match (anchored to the beginning of the string, and the only alternation that can match there):

my (@match) = $str =~ /(?:^foo (?=(?:m \d+ )+bar)|(?<!^)\G)m (\d+) /g;
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... or, in the less-terse form:

my (@match) = $str =~
  / (?: ^foo\ (?= (?:m\ \d+\ )+bar) # overall match from ^foo
      | (?<!^) \G                   # or continue from not-^
    )
    m\ (\d+)\                       # grab each digit sequence
  /xg;
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I think that's the best I got. Match that? :)