From mg.c, the following is the code called after the the value is assigned to the scalar returned by keys %h, sv being that scalar, and MUTABLE_HV(LvTARG(sv)) being the hash:

Perl_magic_setnkeys(pTHX_ SV *sv, MAGIC *mg)
{
    PERL_ARGS_ASSERT_MAGIC_SETNKEYS;
    PERL_UNUSED_ARG(mg);
    if (LvTARG(sv)) {
        hv_ksplit(MUTABLE_HV(LvTARG(sv)), SvIV(sv));
    }
    return 0;
}
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