[...]
use Carp;
use Scalar::Util        qw(openhandle);
[...]

sub read_value {
	my $input  = shift;
	my $length = shift;

	if (ref $input eq 'SCALAR') {
		return _read_value_from_sref($input, $length);
	}
	elsif (defined openhandle($input)) {
		return _read_value_from_fh($input, $length);
	}
	else {
		croak("Invalid usage!\n");
	}
}

sub _read_value_from_fh {
	my $fh     = shift;
	my $length = shift;

	my $curpos     = tell $fh;
	my $filelength = (stat $fh)[7];
	if ($curpos + $length > $filelength) {
		# can not read beyond end of file
		return;
	}

	if (read $fh, (my $value), $length) {
		return $value;
	}
	# don't know how to provoke the else-branch for testing
	else {
		carp("Something bad happened!\n");

		# try to return to previous position
		seek $fh, 0, $curpos;
		return;
	}
}