My best guess is that the excess oxygen ions being generated by the decomposition of the peroxided is reacting and exerting a demand in your COD analysis.  You might try running a blank spiked with approximate levels of peroxide that are present in your samples.  There are test strips available that will give you peroxide levels.  See what kind of COD value that gives you.  You could also try agitating your sample for a few hours prior to COD analysis.  This should drive off any excess peroxide.  Then you can see if the COD values go down.  If you have high chloride levels in your wastewater you may be getting low COD values to begin with.  Then, I think adding the peroxide to your waste may oxidize some of that chloride in solution and liberate it as chlorite or chlorine dioxide.  This would appear to be a higher COD value but is really removal of an interferant.  That is about all the speculation I have.  Does any of this seem totally off base to anyone?
* David S.

Mr David and Jim Royer,

Your suggestions  were applied in determination of COD.
1.The sample was diluted
2.The Chloride(which is more than 1200 ppm) was eliminated
3.Surplus H2O2 was addresses as mentioned in 21st Edition of Standard methods.

With the above implementation, the COD results are reported less than 90 ppm which otherwise were reported as 140 ppm. Further reduction of COD is expected over a period of time.

Thanks a lot to identify our problems and give valuable
tips

Regards,