Use tzset.

use POSIX qw( tzset );

$ENV{TZ} = "America/Los_Angeles";
tzset();
print scalar localtime(time()), "\n";

$ENV{TZ} = 'America/New_York';
tzset();
print scalar localtime(time()), "\n";
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Tue Jul 22 22:02:29 2008
Wed Jul 23 01:02:29 2008
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