$ dd if=/dev/zero of=./test_small_lo count=1 bs=1M
1+0 records in
1+0 records out
$ sudo /sbin/losetup /dev/loop0 ./test_small_lo
$ sudo /sbin/mkfs.ext2 /dev/loop0
 [ snip output ]
$ sudo mount /dev/loop0 /mnt/data
$ df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/loop0                1003        13       939   2% /mnt/data
 [ snip other mounted file systems ]
$ cd /mnt/data
$ sudo touch test_file
$ sudo chmod 666 test_file # So I don't have to run code below as root

$ perl -e '
> open( FH, ">", "test_file" ) or die $!;
> for( 0 .. 10_000_000 ) { print FH 1 or die $! }
> close(FH) or die $!
'
No space left on device at -e line 3.