If your present, ordinary HDMI cable simply does not fit behind your new flat screen TV, you can find a new, angled HDMI cable with an angle suiting your purposes here, or you can take advantage of an angle adapter which you place on the end of your cable, which now fits the situation.

The HDMI angle cables and adapters are available in variants with fixed angles and variable angles, so that the cable fits all situations. Now, it's just about finding the right length.