for (;;) {
  undef $!;
  unless (defined( $line = <> )) {
    die $! if $!;
    last; # reached EOF
  }
  # ...
}

##</code><code>##

#!/usr/bin/perl 
sub doit {
   open FH, shift or die "open failure: $!\n";
   $! = shift;
   1 while <FH>;
   $n = $!+0;
   print "n=$n s=$!\n";
}
doit $0, 5;   
doit $0, 6;  
doit $0, undef;

##</code><code>##

n=5 s=I/O error
n=6 s=No such device or address
n=0 s=