In such cases, I usually prefer to use exceptions, i.e. die / eval.

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

my $string;
eval {
  $string = foo();
};
if ( $@ ) {
  ( my $code, $string ) = split /\n/, $@;
  ...
}
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If you really don't want to go that route, then you could return a return "object" which contains multiple fields. (It doesn't have to be a blessed object, though.)

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

my( $code, $string ) = foo();
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Lastly: realize that die can return an object, not just a string. This can be pretty useful in cases such as this.

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

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