try this:

  my $already = exists $ENV{TZ};
  my $original_time_zone = $ENV{TZ} if $already;
  $ENV{TZ} = 'some other time zone';
  $string_time = POSIX::strftime("%Y-%m-%d %H:%M:%S", localtime($epoch_time));
  $ENV{TZ} = $original_time_zone if $already;
  delete $ENV{TZ} unless $already;

it's seems crude, but temporarily changing the time zone for the system was what it took. (for me it was just an apache child)