Thank you for the report. I confirm I can reproduce the issue.

The inet_aton() function is provided by Socket Perl module and it indeed calls gethostbyname() that is not thread-safe:

void
inet_aton(host)
    char *  host
    CODE:
    {
    struct in_addr ip_address;
    struct hostent * phe;

    if ((*host != '\0') && inet_aton(host, &ip_address)) {
        ST(0) = sv_2mortal(newSVpvn((char *)&ip_address, sizeof(ip_address)));
        XSRETURN(1);
    }

→   phe = gethostbyname(host);
    if (phe && phe->h_addrtype == AF_INET && phe->h_length == 4) {
        ST(0) = sv_2mortal(newSVpvn((char *)phe->h_addr, phe->h_length));
        XSRETURN(1);
    }

    XSRETURN_UNDEF;
    }

As a workaround I recommend using getaddrinfo() from the same Socket module instead. It's thread safe and supports IPv6. As replacement for IO:Socket::INET I recommend IO::Socket::IP instead.

Created attachment 1555175 [details]
Proposed fix ported to 2.010

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2020:0997