IO::File is only involved if you use HANDLE->print(...), not for print(HANDLE ...), much less before the handle is even created by open. It's not relevant here.

Contrary to what you said, subs works fine.

use strict;
use warnings;
use subs qw( open );
use Errno qw( EPIPE );

sub open {
   $! = EPIPE;
   return 0;
}

open(my $fh, '<', \my $buf) or die("open: $!\n");
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open: Broken pipe
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Update: You probably want to test a module without changing it, so your code would look more like the following:

package ModuleToTest;

sub f {
   open(my $fh, '<', \my $buf) or die("open: $!\n");
}

1;
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use Test::More tests => 1;

{
   package ModuleToTest;
   use subs qw( open );
   use Errno qw( EPIPE );
   sub open {
      $! = EPIPE;
      return 0;
   }
}

use Module;

ok( !eval { Module->f(); 1 } );
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1..1
ok 1
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