$ cat > XXX qwe rty ##

##

$ perl -e 'while (<>) { print }' XXX
qwe
rty

##

##

$ perl -e 'while () { print }' XXX
^C

##

##

$ perl -e 'while () { print }' XXX
typed
typed
from
from
keyboard
keyboard

##

##

$ perl -e 'while () { print }' < XXX
qwe
rty

##

##

$ cat XXX | perl -e 'while () { print }'
qwe
rty