The use of bareword filehandles vs .indirect filehandles is discussed in detail in Perl Best Practices. It says:

One of the most efficient ways for Perl programmers to bring misery and suffering upon themselves and their colleagues is to write this:

    open FILE, '<', $filename
        or croak "Can't open '$filename': $OS_ERROR";
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Using a bareword like that as a filehandle causes Perl to store the corresponding input stream descriptor in the symbol table of the current package. Specifically, the stream descriptor is stored in the symbol table entry whose name is the same as the bareword; in this case, it's *FILE. By using a bareword, the author of the previous code is effectively using a package variable to store the filehandle.

If that symbol has already been used as a filehandle anywhere else in the same package, executing this open statement will close that previous filehandle and replace it with the newly opened one. That's going to be a nasty surprise for any code that was already relying on reading input with <FILE>

The advice is

Use indirect filehandles.

    open my $FILE, '<', $filename
        or croak "Can't open '$filename': $OS_ERROR";
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Perl Best Practices is a controversial book, but I don't think this section is particularly debated.