use warnings;
use strict;
use Scalar::Util qw/blessed/;

@Error::ISA = qw//; # "base class" for errors
@BadInput::ISA = qw/Error/;
sub doit {
	my $in = shift;
	if ( $in =~ /foo/ ) {
		return bless [123,456], 'RealReturnValue';
	}
	else {
		return bless ['abc','def'], 'BadInput';
	}
}

for my $val ("foobar", "quzbaz") {
	my $ret = doit($val);
	if ( my $err = blessed $ret and $ret->isa('Error') ) {
		print "$val -> Error: $err\n";
	}
	else {
		print "$val -> OK\n";
	}
}

__END__

foobar -> OK
quzbaz -> Error: BadInput