The bug reports on CPAN for List::Util don't list it, but I looked at the source code, and found the following C code for reduce:

void
reduce(block,...)
    SV * block
PROTOTYPE: &@
CODE:
{
    SV *ret = sv_newmortal();
    int index;
    GV *agv,*bgv,*gv;
    HV *stash;
    CV *cv;
    OP *reducecop;
    PERL_CONTEXT *cx;
    SV** newsp;
    I32 gimme = G_SCALAR;
    U8 hasargs = 0;
    bool oldcatch = CATCH_GET;

    if(items <= 1) {
    XSRETURN_UNDEF;
    }
    agv = gv_fetchpv("a", TRUE, SVt_PV);
    bgv = gv_fetchpv("b", TRUE, SVt_PV);
    SAVESPTR(GvSV(agv));
    SAVESPTR(GvSV(bgv));
    GvSV(agv) = ret;
    cv = sv_2cv(block, &stash, &gv, 0);
    reducecop = CvSTART(cv);
    SAVESPTR(CvROOT(cv)->op_ppaddr);
    CvROOT(cv)->op_ppaddr = PL_ppaddr[OP_NULL];
#ifdef PAD_SET_CUR
    PAD_SET_CUR(CvPADLIST(cv),1);
#else
    SAVESPTR(PL_curpad);
    PL_curpad = AvARRAY((AV*)AvARRAY(CvPADLIST(cv))[1]);
#endif
    SAVETMPS;
    SAVESPTR(PL_op);
    SvSetSV(ret, ST(1));
    CATCH_SET(TRUE);
    PUSHBLOCK(cx, CXt_SUB, SP);
    PUSHSUB(cx);
    if (!CvDEPTH(cv))
        (void)SvREFCNT_inc(cv);
    for(index = 2 ; index < items ; index++) {
    GvSV(bgv) = ST(index);
    PL_op = reducecop;
    CALLRUNOPS(aTHX);
    SvSetSV(ret, *PL_stack_sp);
    }
    ST(0) = ret;
    POPBLOCK(cx,PL_curpm);
    CATCH_SET(oldcatch);
    XSRETURN(1);
}
[download]

Now the problem is that I may know how to read C, but I sure don't know XS. My wild guess is that the

      if (!CvDEPTH(cv))
       (void)SvREFCNT_inc(cv);
[download]

should be matched with a decrement (most likely SvREFCNT_dec) to prevent memory leaking. But quite possibly, the problem might lie elsewhere in the XS code. I think that you best report the bug to the author via RT, and include your test snippet or a link to this thread.