For that, you'd want to use the global (/g) modifier on the end of your regex, and use the regex in a list context:
@ids = m/:ID\s\d*/g

You'll need to do another regex to pull the P_IDs,
@p_ids = m/:P_ID\s\d*/g,
and you won't be able to use backreferencing to ditch the ":ID " part.

For that, I guess you could use another regex, or, if the data is regular, something like:
for $i (0..$#ids) {$ids[Ģi] = substr ($ids[$i], 4);}