# Text wrapped to fit in the screen
my $text = "0011222STRUCTdata..........6ab.." .
           "02121STRUCTdata.........." .
           "021232STRUCTdata......" .
           "02342STRUCTdata......";

my @matches = $text =~ m/(\d+)          # First match some numbers
                         STRUCT         # Then the STRUCT keyword 
                         (.+?)          # Then the data, that have to 
                                        # be followed by 
                         (?=            # (look-ahead assertion)
                          (?:\d+STRUCT  #    - numbers and 'STRUCT'
                                |       #  or
                             $          #    - end of line
                          )
                         )/xg;

# Then remove the first two matches that is first numbers and 
# first data
@matches = splice @matches, 2;

{
  local $, = ",";
  print @matches, "\n";
}