Shell Programming and Scripting

awk '
{split($NF,c,",")}      # split the last field ($NF) into array c (c[1]=x, c[2]=y)
!(c[1]%2) {             # use the modulo operator % to search for lines with an even x
  a[++i]=$0             # if x is even assign the line into array a
  next                  # read the next line; these lines never reach the next commands!
}
i{for(j=i;j>0;j--){     # for the next first line with an odd x (i is not 0)
    print a[j]          # .. print array "a" reversed, before the current line
  }
  i=0                   # .. and reset i
}
{print}                 # print the line with an odd x
END{                    # after the last line, check the value of i
  if(i){                # .. if i > 0 the last line is a line with an even x
    for(j=i;j>0;j--){   # .. array a is not printed yet
      print a[j]        # .. print array a
    }
  }
}
' starting_file > new_file

use strict;
use warnings;
my @sort_keys;
open (my $IN, 'd.dat') or die "$!"; 
my @lines = <$IN>;
close $IN;
open (my $OUT, ">", 'out.dat') or die "$!";
foreach my $line (@lines) {
   my ($x,$y) = $line =~ / (\d+),(\d+)\s*$/;
   if ($x % 2) {
      if (@sort_keys) {
         print $OUT map {$_->[1]} sort {$b->[0] <=> $a->[0]} @sort_keys;
         @sort_keys = ();
         print $OUT $line;
      }
      else {
         print $OUT $line;
      }
   }
   else {
      push @sort_keys, [$y, $line];
   }
}
close $OUT;