#! /usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my $filename = q{monk.txt};
open my $fh, q{<}, $filename
  or die qq{cant open *$filename* to read: $!\n};

my (@db, @records);
while (my $line = <$fh>){
  chomp $line;
  if ($line =~ /BEGIN_GROUP VARIABLE,PM2\.5/ .. $line =~ /END_GROUP/){
    push @records, $line;
    if ($line =~ /END_GROUP/){
      push @db, [@records];
      #warn Dumper \@db;
      @records = ();
    }
  }
}

for my $group (@db){
  for my $record (@{$group}){
    print qq{$record\n};
    # split on comma to get the field
    # you need?
  }
  print q{*} x 10, qq{\n};
}