use strict;
use warnings;

my $file_name = 'G:\\I386\\DRIVER.CAB';  # 50MB file.

open(my $fh, '<', $file_name)
   or die("Unable to open file \"$file_name\": $!\n");

# Read 100 bytes at a time.
#binmode($fh);
#$/ = \100;

my $line;

undef $!;
$line = <$fh>;
my $ok_defined = defined($!);
my $ok_str_val = "$!";
my $ok_num_val = 0+$!;

print("Eject the media then press Enter.");
<STDIN>;

undef $!;
for (;;) {
   undef $!;
   last if not defined ($line = <$fh>);
   warn(length($line), "\n");
}
my $bad_defined = defined($!);
my $bad_str_val = "$!";
my $bad_num_val = 0+$!;

close($fh)
   or warn("Unable to read file \"$file_name\": $!\n");

print("On success, ");
if ($ok_defined) {
   print("\$! = \"$ok_str_val\"/$ok_num_val\n");
} else {
   print("\$! is not defined\n");
}

print("On failure, ");
if ($bad_defined) {
   print("\$! = \"$bad_str_val\"/$bad_num_val\n");
} else {
   print("\$! is not defined\n");
}