From "perldoc perlvar" on the $! variable:

If used numerically, yields the current value of the C "errno" variable, with all the usual caveats. (This means that you shouldn't depend on the value of "$!" to be anything in particular unless you've gotten a specific error return indicating a system error.) If used an a string, yields the corresponding system error string. You can assign a number to "$!" to set errno if, for instance, you want ""$!"" to return the string for error n, or you want to set the exit value for the die() operator.

The emphasis is mine, but I left it in context so you could see what's happening here. $! is a special variabl that returns slightly different information depending upon the context. Further, though not stated explicitly, you cannot assign a string to it.

Now, regarding your code: it can't be OO because the object must be a reference to something (see "perldoc -f bless" for more information. However, I'll assume for the moment that you know that and just wrote $self ne 'hm' for testing. The real point that I wanted to get to is that if you're going to use OO code, you should provide OO methods for dealing with errors:

package Foo;
use strict;

sub new {
  my $class = shift;
  bless {
    _errno  => 0,
    _errmsg => '',
    _foo    => undef
  }, $class;
}

sub set_foo {
  my ( $self, $foo ) = @_;
  if ( $foo <= 0 ) {
    @{$self->{qw/_errno _errmsg/}} = ( 1, 'foo must be positive' );
    return;
  }
  else {
    $self->{_foo} = $foo;
    return 1;
  }
}

sub error  { $_[0]->{_errno}  }
sub errmsg { $_[0]->{_errmsg} }
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With the above (untested) code, the object keeps track of errors internally. Then, you can just do something like this:

use Foo;
my $thing = Foo->new;
my $success = $thing->set_foo( -2 );

if ( $success ) {
  # do something
}
else {
  # you can also do if ( $thing->error ) {
  die $thing->errmsg;
}
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There are many different ways to handle that, but I think you get the idea. Good luck!

Cheers,
Ovid

Update: I changed the error return to a blank return (i.e., return nothing). I had it set to "return 0", but if returned into an array, this will cause the array to evaluate as true. Whoops! :)

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