Consider this example.

use strict;
use warnings;
use Fatal qw(:void open);

# set $! to some random number; we just want to see
# if it is modified
$! = 42;
print "$!\n";

open my $f, '<no_such_file' or die "Can't open no_such_file: $!";
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The output is as follows.

Illegal byte sequence
Can't open no_such_file: Illegal byte sequence at t.pl line 10.
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$! doesn't seem to get modified. Taking a look at Fatal.pm, one sees the following at about line 121:

local(\$", \$!) = (', ', 0);
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$" and $! are localized in the replacement subroutine, open in this example. Removing above line, and running the example again yields the following.

Illegal byte sequence
Can't open no_such_file: No such file or directory at t.pl line 10.
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This seems to be the desired outcome.

I don't use Fatal, so I can't tell if the localized $! is a bug or a feature.