use POSIX qw( tzset );

print localtime(time) . " in $ENV{TZ}\n";

$ENV{TZ} = 'GMT-8';
tzset();

print localtime(time) . " in $ENV{TZ}\n";

##</code><code>##

Mon Dec  3 00:22:29 2007 in America/New_York
Mon Dec  3 13:22:29 2007 in GMT-8