It works fine here, mIRC 6.16.
Bear in mind, if you have something like the following in a file (info.txt for example):
Testing
Blah test
Moo
Then use /write -dstest info.txt it won't delete anything, because the word 'Test' is not there - the word 'Testing' is. You need to use the -w# switch, for example, /write -dw*test* info.txt, in which case it will remove Testing.
Regards,