#!/usr/bin/perl -wl
use strict;
use Inline C => <<'END_C';

SV* GenerateSessionID() {

    char uuid[16];    // buffer

    // fill buffer, i.e. mimic your XSessionID()
    int i;
    for (i=0; i<16; i++) {
        uuid[i] = 0xA0 + i;
    }

    return newSVpvn(uuid, 16);
}
END_C

my $uuid = GenerateSessionID();
print join ' ', unpack("(H2)*", $uuid);

##</code><code>##

$ ./962624.pl
a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 aa ab ac ad ae af

##</code><code>##

SV *
GenerateSessionID()
    PREINIT:
        char uuid[16];
    CODE:
        XSessionID((uuid_t*)uuid);
        RETVAL = newSVpvn(uuid, 16);
    OUTPUT:
        RETVAL

##</code><code>##

void
GenerateSessionID()
    PREINIT:
        char uuid[16];
    PPCODE:
        XSessionID((uuid_t*)uuid);
        ST(0) = newSVpvn(uuid, 16);
        sv_2mortal(ST(0));
        XSRETURN(1);