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$ cat > XXX
qwe
rty

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

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

$ perl -e 'while (<STDIN>) { print }' XXX
typed
...
from
keyboard
keyboard

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

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