a small tweak would be to use the  $^N pre-defined variable that "contains whatever was matched by the most-recently closed group (submatch)".
this makes the sub-expression less sensitive to its position in a possibly complex regex expression.

perl -wMstrict -e "my $re = qr/ ^ (\d+?) ( (??{ qq(.{$^N}) }) ) /x;
/$re/, print qq($_ [$1] [$2] \n) for @ARGV"
43fffxxx 54321234 12xxxxxxxxxxx 12xxxxxxxxxxxx
43fffxxx [4] [3fff]
54321234 [5] [43212]
12xxxxxxxxxxx [1] [2]
12xxxxxxxxxxxx [1] [2]
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