sub sep_id {
...

         open (TMPFILE, "> $tmp_file") or (die "Could not open $tmp_file: $!");
         select TMPFILE;
         foreach $water (keys %wat_freq) {
            $freq=0;
            $code=undef;
            if ($wat_freq{$water} >= $C_NETSIZE) {
               @ids=();
               foreach (@info) {
                  if (substr($_,13,5,) == $water ) {
                     push @ids, substr($_,8,5);
                  }
               }
               @ids=sort {$a <=> $b} @ids;
               $code = join ":", $water . "+", @ids;
               $code =~ s/ /0/g;
               printf "%02d:+%s:\n",$wat_freq{$water}, $code;
            }
         }
         select STDOUT;

##</code><code>##

  my $SOME_GLOBAL_VARIABLE = 0;
  some_sub_with_side_effects();

  ...
  sub some_sub_with_side_effects {
    $SOME_GLOBAL_VARIABLE = 1;
  }

##</code><code>##

  my $SOME_GLOBAL_VARIABLE = some_sub_without_side_effects();

  ...
  sub some_sub_without_side_effects {
    return 1;
  }