Your code captures and assigns to $max_id only the last one of the first group of digits found. I expect that's probably not what you wanted.

perl -Mstrict -W -E 'say $1 if "hello 0 and 123 goodbye" =~ /(\d)+/;'
0
[download]

$ perl -Mstrict -W -E 'say $1 if "hello and 123 goodbye" =~ /(\d)+/;'
3
[download]

Perhaps you want:

$ perl -Mstrict -W -E 'say $1 if "hello and 123 goodbye" =~ /(\d+)/;'
123
[download]

Hope this helps!

(Update: show suggested fix and additional failing example)