open '/FILE', file;
print </FILE>

__END__

open /FILE, 'file';
use File::Glob ();
print glob('/FILE');
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• glob

You can't have a slash in a filename for the same reason you can't have one in a sub name:

sub hello/world {
  say "Hello World";
}
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Subs and non-lexical filehandles are declared using a "bareword", and barewords can't contain slashes. A better question is why in the name of all that is good and holy would you want to do this?!

You do know that the file handle and the file name have nothing to do with each other?

OK, I'll admit I lied earlier. It is possible to name a sub with a slash. You just need to declare it and call it using unusual syntax...

*{'hello/world'} = sub {
    print shift, "\n";
};

&{'hello/world'}("Hello World!");
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And similarly it can be done with filehandles...

open *{'hello/world'}, '>', '/home/tai/tmp/helloworld.txt';
print {*{'hello/world'}} "Hello World!\n";
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The "why in the name of all that is good and holy" question still stands though.

Nowadays there's rarely any excuse to not be using lexical file handles. (i.e. file handles declared using the my keyword.)

And similarly it can be done with filehandles...

Like the OP shows, the globularness isn't required, just quote a string

open '/ugly', 'foo.txt' or die $!;
print readline '/ugly';
close '/ugly';
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Because that is how <> diamond operator works, it wants a bareword if you want readline , otherwise you get glob

But, you should never use arbitrary strings as filehandles, its just painful for the sake of being painful (a stupid idea)