:~$ cat > OOPS
3
2
1
0^D 

:~$ perl -e '$foo = 42; while($foo and $_ = <>){print}' OOPS
3
2
1
:~$ 

:~$ perl -e '$foo = 42; while($foo and defined($_ = <>)){print}' OOPS
3 
2
1
0:~$