this XS proved to work:

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "ppport.h"

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

MODULE = tztime         PACKAGE = tztime

long
tz_mktime(sec, min, hour, mday, mon, year, tz_name)
  int sec
  int min
  int hour
  int mday
  int mon
  int year
  char *tz_name

    CODE:

      long epoch;
      struct tm tz_tm;
      char *old_tz;
      const char *tz_p;
      bool restore = FALSE;

      tz_tm.tm_sec = sec;
      tz_tm.tm_min = min;
      tz_tm.tm_hour = hour;
      tz_tm.tm_mday = mday;
      tz_tm.tm_mon = mon;
      tz_tm.tm_year = year;
      tz_tm.tm_wday = 0;
      tz_tm.tm_yday = 0;
      tz_tm.tm_isdst = -1;

      tz_p = getenv("TZ");
      if(tz_p != NULL)
      {
        restore = TRUE;
        old_tz = malloc(strlen(tz_p) + 1);
        if(old_tz == NULL) {
          XSRETURN_UNDEF;
        }
        strncpy(old_tz,tz_p,strlen(tz_p) + 1);
      }

      setenv("TZ",tz_name,1);
      tzset();
      epoch = (long)mktime(&tz_tm);

      if(restore) {
        putenv(old_tz);
        free(old_tz);
      } else {
        unsetenv("TZ");
      }
      tzset();

      RETVAL = epoch;

    OUTPUT:
      RETVAL

void
tz_gettime(epoch, tz_name)
  long epoch
  char *tz_name;

    PPCODE:

      struct tm tz_tm;
      char *old_tz;
      const char *tz_p;
      bool restore = FALSE;

      tz_p = getenv("TZ");
      if(tz_p != NULL)
      {
        restore = TRUE;
        old_tz = malloc(strlen(tz_p) + 1);
        if(old_tz == NULL) {
          XPUSHs(sv_2mortal(newSVnv(errno)));
        }
        strncpy(old_tz,tz_p,strlen(tz_p) + 1);
      }

      setenv("TZ",tz_name,1);
      tzset();
      localtime_r(&epoch,&tz_tm);

      if(restore) {
        putenv(old_tz);
        free(old_tz);
      } else {
        unsetenv("TZ");
      }
      tzset();

      XPUSHs(sv_2mortal(newSVnv(tz_tm.tm_sec)));
      XPUSHs(sv_2mortal(newSVnv(tz_tm.tm_min)));
      XPUSHs(sv_2mortal(newSVnv(tz_tm.tm_hour)));
      XPUSHs(sv_2mortal(newSVnv(tz_tm.tm_mday)));
      XPUSHs(sv_2mortal(newSVnv(tz_tm.tm_mon)));
      XPUSHs(sv_2mortal(newSVnv(tz_tm.tm_year)));
      XPUSHs(sv_2mortal(newSVnv(tz_tm.tm_wday)));
      XPUSHs(sv_2mortal(newSVnv(tz_tm.tm_yday)));
      XPUSHs(sv_2mortal(newSVnv(tz_tm.tm_isdst)));
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