seq 9 > test.txt
ls -l test.txt
perl -we 'open( FILE, "< ./test.txt" ) && truncate( FILE, 8 ) && close(FILE);'

$ ls -lgG test.txt
-rw-rw---- 1 18 2013-08-08 09:49 test.txt

perl -we 'open( FILE, "< ./test.txt" ) && truncate( FILE, 8 ) && close(FILE);'

perl -we 'open(FILE, "<", "./test.txt") or die "open: $!";
          truncate(FILE, 8) or die "truncate: $!";
          close(FILE);'

truncate --size=1G test.txt