I should have said this before to make it a little clearer.

Read the contents of $! to glean the exact cause of an error right after an unsuccessful operation, Do not test $! to determine the success of an operation, use the TRUE/FALSE value returned instead. The Menmonic of $! is -- "What just went bang?" not "Did I just hear bang?", There is an obvious difference.

Your reading of the operation concerning $! is definitely right but the reading concerning the context in which it is done is not. Here, if <> returns undef to $line, defined returns FALSE to unless () which puts the execution control in its block, it is there and only there where $! is current and valid.

E.g 
 unless (defined( $line = <> )) {
      die $! if $!;      # $! is only current and valid in the afterma
+th of a failure
      last; # reached EOF
 }

[download]
is not the same as 
$line = <>;
if ($!) {
  print "Ohh noo, $!";  # an indefinite false-positive
}

[download]
if you should ask why?? Consider the following.. 
$ touch test;  # create the dummy

$ perl -Mstrict -Wle 'undef $!; open my $fh, "<", "test" or warn "warn
+ : $!"; print "after : $!"'
after : Inappropriate ioctl for device

$ rm test;

$perl -Mstrict -Wle 'undef $!; open my $fh, "<", "test" or warn "warn 
+: $!"; print "after : $!"
warn : No such file or directory at -e line 1.
after : No such file or directory

[download]
In the first operation, open succeeds and returns a TRUE value so warn() is skipped. However in the second, open fails and returns FALSE, warn is triggered spitting out a valid $!. The $! outside the expression is not trustworthy as one would infer from the documentation of $! in perlvar.

update

a readline error can be distinguished by undefining $! before the call

undefining $! is a useless, almost redundant operation IMO, consequetive operations will set $! if they need to, sometimes regardless of whether they succeed or fail.

checking if $! is set to non-0

It's more like, checking if $! is TRUE, because FALSE can be representation of not just 0 but undef, "" (blank) or "0";

In reply to Re^3: Best way to handle readline errors? by Firefly258
in thread Best way to handle readline errors? by jrw

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