This should be documented in localtime, but POSIX's tzset updates the timezone used by localtime. See the documentation for the underyling C library function.

It's not really designed as a tool to render times in other arbitrary time zones. You may want to use a module such as DateTime for that.

$ perl -MDateTime -le'
   my $dt = DateTime->from_epoch( epoch => 1234567890 );
   for ("America/New_York", "Europe/Rome") {
      $dt->set_time_zone($_);
      print $dt->strftime("%Y/%m/%d %H:%M:%S");
   }
'
2009/02/13 18:31:30
2009/02/14 00:31:30
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