sub foo {
  open F, ...
    or die "-1\nerror opening file";
  return $result;
}

my $string;
eval {
  $string = foo();
};
if ( $@ ) {
  ( my $code, $string ) = split /\n/, $@;
  ...
}

##</code><code>##

sub foo {
  open F, ... or 
    return( [ -1, "error opening file" ] );
  return( [ 0, $result ] );
}

my( $code, $string ) = foo();
 

##</code><code>##


sub foo {
  open F, ...
    or die [ -1, "error opening file" ];
  return $result;
}

my $string;
eval {
  $string = foo();
};
if ( $@ ) {
  ( my $code, $string ) = $@;
  ...
}