Get rid of the space after the 3. It was added to by the CB line breaker. And by the way, the parens around \d are useless and slow down the match.

print grep /^(?:[^,]*,){4},{3}\d,0,\d/, <DATA>; 

__DATA__
1,2,0,First Test,,,,0,0,7,,,,,,,,,,,
1,2,0,Starting madvise bss tests,1,buffer,G,1,1,0,Y,,,P,G,,,,,,
1,2,0,Starting madvise bss tests,2,buffer,G,1,2,0,Y,,,P,G,,,,,,
1,2,0,Starting madvise bss tests,3,buffer,G,1,3,0,Y,,,P,G,,,,,,
1,2,0,Second Test,,,,0,0,7,,,,,,,,,,,
1,2,0,Starting madvise bss tests,1,buffer,G,1,1,0,Y,,,P,G,,,,,,
1,2,0,Starting madvise bss tests,2,buffer,G,1,2,0,Y,,,P,G,,,,,,
1,2,0,Starting madvise bss tests,3,buffer,G,1,3,0,Y,,,P,G,,,,,,
[download]

1,2,0,First Test,,,,0,0,7,,,,,,,,,,,
1,2,0,Second Test,,,,0,0,7,,,,,,,,,,,
[download]