# We're hunting the (properly bracketed)
$str =~         # 'm \d+' occurrences.  They must be 
  / (?:^foo\s   # - proceeded by and initial foo
    |           #  OR
    (?<!^)\G)   # - the end of a previous successful match
                # - but not the beginning of the string
     m \s (\d+)  # Here's the guy we really want.
                # But he must be followed by the right stuff
    (?=         # Lookahead says he must be followed by:
     (?:m \s \d+ \s)*   # Any number of m \d+ groups.
     bar)/xg;   # Finally terminated with bar (though not
                #  necessarily the end of string.)


##</code><code>##

  - starts at the current 'beginning' of the string
  - matches the 'm \d+' at the current position
  - matches (fore) the foo and all the 'm \d+' before the current position
  - matches (aft) all the remaining 'm \d+' and the final bar 
     -  and THROWS AWAY the fore and aft matches  (they're non-capturing)