Perl may not be "evil", but there have been circumstances in the past where something quite along these lines had occured, but with the C compiler (!) instead:

1984 Ken Thompson's CC Hack¹

Ken Thompson described this virus² as "the cutest program I ever wrote", and since his account of it appeared in 1984, it has become folk law of the information age. Ken modified the source of the C compiler (CC) such that the compiler would detect when it was compiling the UNIX "login" command and hence produce a compromised version of the login command which would accept either a valid password from the system password file, or a particular known password. Effectively giving him a back door to any system whose "login" binary was compiled with his compromised C compiler. This in itself results in a curious situation, despite all attempts to verify the source of the "login" command, it is impossible, once the C compiler has been compromised to build a secure "login" binary.

Not content with this level of subversion, Ken introduced a second modification to the compiler source such that it would detect when it was compiling a new version of itself. It would then incorporate both modifications into the new compiler binary.

This event came as quite a shock to the UNIX community. They were taken completely by surprise.

In all honesty, though, even if you read through every single line of Perl's interpreter code, do you think you could find malicious code even if it were in there? Not likely. It's not like it will have a giant comment beside it that would make it stand out:

/* Here is the EVIL VIRUS CODE that is implanted in every Perl
   Absolutely no peeking. This is not here. */
if (moon == color_blue && day_of_week == 5 && day == 13)
    delete_hard_drive(completely);
[download]

In fact, that hypothetical code would look just like any of the thousands of other lines of code. You would never know.

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1. Excerpt taken from Linux and the Virus Threat by Shaun Cloherty (Feb 4, 2001)
2. K. Thompson, "Reflections on trusting trust", Communication of the ACM, Vol. 27, No. 8, August 1984, pp. 761-763.