Answer: No. Only someone who actually testifies under oath can commit perjury. If a person never testifies under oath, he cannot commit perjury no matter how many lies he may tell. A lie that is not told under oath is simply a lie.
Testifying under oath includes testifying before a grand jury, at a Congressional or legislative committee hearing, at family or bankruptcy court, or in a sworn statement or affidavit. Without testimony sworn under oath, there can be no perjury.
However, people who tell lies that harm another person may have committed other wrongs for which they could be sued for damages. For example, if this witness had told lies about you to your mutual acquaintances, members of your community, or co-workers, and the lies damaged your reputation, that may be defamation and you could sue the liar. Also, a person who lies to an investigator may be charged with a crime.
Another twist to the law about perjury and testimony under oath is that a person who is harmed as a result of lies told by a witness during sworn testimony (for example, convicted of a crime or required to pay civil damages) cannot sue the witness for the harm done. An example of this would be if the witness had agreed to testify under oath and then told lies on the stand that caused you to lose your case. While this testimony would be perjury, you could not sue the witness because of it. The reason for this is that our legal system greatly values witness testimony; the risk of having to pay civil damages because of sworn testimony may discourage people from coming forward and testifying freely. This could severely disrupt the legal system, because it relies so heavily on sworn testimony.
Of course, anyone who lies under oath may be charged with the crime of perjury, which carries a possible prison sentence and fine. For more on perjury, see Perjury.