You handle STDIN wrong (you need a filehandle or a typeglob - not a string) and it is usually better to use lexical variables for filehandles.

One way of doing it would be:

use IO::File;

my $handle = $FILE
               ?   do { IO::File->new($FILE) or fatal_error(".."); }
               :   *STDIN;


# later you can use <$handle>
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