The issue comes from assigning $foo to the string 'FILE'. As others have mentioned, you can call open on an undefined scalar lvalue and it will magically assign a filehandle to it. If you are so inclined, you can also do it explicitly yourself:

use IO::File;
my $foo = new IO::Handle;
open $foo, '<', 'data.txt';
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Update: switched modules per ikegami's suggestion.