Running DOS inside C#

[Shuja Ali]

It is not that you can just execute Internal Commands, you can execute any EXE/COM file present on your system, provided the path of that file is set-up in Environment Variable (Path)..

For your code

p.StandardInput.WriteLine(@"perl extract.pl test.htm > test.out");

Setup up the Path of perl file and then try!
Or
Just change the directory to the location where your files exist and then run your Perl compiler/command...
Something like this

p.StandardInput.WriteLine(@"chdir \MyPerlDirectory");

And then run the above command...

HTH