The Shulhan Aruch (Orah Haim 32:5; listen to audio recording for precise citation) writes that a Sofer must use his right hand when writing Tefillin. Even if he is ambidextrous, the text must be written with the right hand. The Shulhan Aruch adds that Tefillin written with one’s left hand is not valid for the Misva, though if no other Tefillin are available one may wear Tefillin written with the Sofer’s left hand.
Hacham David Yosef, in his Halacha Berura, notes that a Sofer who is left-handed and performs all tasks with his left hand must write Tefillin with his left hand, and if he wrote the text with his right hand, the Tefillin are not valid for use.
Hacham David notes an incident that occurred in Egypt where it was a discovered that a man who did not have hands was writing Tefillin by holding the quill in his mouth. The Poskim rule that Tefillin written in this fashion are invalid for use, even if one has no access to any other Tefillin.
It goes without saying that a photocopied text is invalid for the Misva of Tefillin.
Unfortunately, there are some unscrupulous people who want to make easy money by photocopying the Tefillin script using advanced technology, such that the copy is a precise replica of the original. For this and other reasons, it is incumbent upon everyone purchasing Tefillin to ensure to make his purchase from a trustworthy Sofer. Even if the script looks perfect, nobody can tell whether the Sofer wrote it properly and in accordance with all the relevant Halachic details. One must ensure to purchase Tefillin from a knowledgeable and G-d-fearing Sofer so he can be confident that he fulfills this Misva as required by Halacha.
Summary: A right-handed Sofer must write Tefillin with his right hand, and a left-handed Sofer must write Tefillin with his left hand. If the wrong hand was used for the writing, the Tefillin may not be used for the Misva. It is therefore crucially important to ensure to purchase Tefillin only from a knowledgeable and G-d-fearing Sofer.
