Question

Is it OK to assign to $! on an error in Perl?

E.g.,

if( ! (-e $inputfile))
{
      $! = "Input file $inputfile appears to be non-existent\n";
      return undef;
}

This way I can handle all errors at the top-level.

Thanks.

Answer 1

If you assign to $!, it is placed in the system errno variable, which only takes numbers. So you can in fact do:

use Errno "EEXIST";
$! = EEXIST;
print $!;

and get the string value for a defined system error number, but you can’t do what you want – setting it to an arbitrary string. Such a string will get you a Argument “…” isn’t numeric in scalar assignment warning and leave errno set to 0.

The other problem is that $! may be changed by any system call. So you can only trust it to have the value you set until you do a print or just about anything else. You probably want your very own error variable.