From the perlapi manpage (emphasis mine):

eval_pv Tells Perl to "eval" the given string and return an SV* result.

I tried using

        SV* ret = eval_pv("1;", TRUE);
        SvREFCNT_dec(ret);
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but that didn't work (probably because eval_pv works "kindof like a subroutine" in that it returns a mortal SV* - or something; I confess ignorance of the subtleties in perlembedding) - so messed around with the code until I came up with this:

    for (;;) {
        ENTER;
        SAVETMPS;
        SV* ret = eval_pv("1;", TRUE);
        FREETMPS;
        LEAVE;
    }
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Which doesn't leak for me (perl, v5.8.5 built for i686-linux-thread-multi).

update: this is not really documented AFAIK, probably because you're not likely to be calling eval_* all that much. The perlembed documentation should probably mention something about how to deal with the return value (as it does describe handling the various call_* parameters and return values).