Seems to be a limitation of Perl that it only honours the setting at startup. If you change it within your script and spawn a child, then the spawned Perl sees and honours the change.

P:\test>perl -le"print scalar localtime; $ENV{TZ}='PST8PDT'; print sca
+lar localtime; system 'perl -le \"print scalar localtime\"' "
Thu Jan 19 19:52:12 2006
Thu Jan 19 19:52:12 2006
Thu Jan 19 11:52:12 2006
[download]

According the CRT docs, it has an equivalent of the POSIX tzset() call (_tzset), but neither the tzset() exported by the POSIX module, nor calling the CRT _tzset() after having set the TZ within the program, has any affect upon the times returned by localtime within the current process.

However, running this

#include <windows.h> 
#include <stdio.h> 
#include <stdlib.h>
#include <time.h>
 
 
DWORD main(int argc, char *argv[]) { 
    struct tm *newtime;
    char am_pm[] = "AM";
    __time64_t long_time;

    _putenv( argv[1] );

    _time64( &long_time );                /* Get time as long integer.
+ */
    newtime = _localtime64( &long_time ); /* Convert to local time. */

    if( newtime->tm_hour > 12 )        /* Set up extension. */
            strcpy( am_pm, "PM" );
    if( newtime->tm_hour > 12 )        /* Convert from 24-hour */
            newtime->tm_hour -= 12;    /*   to 12-hour clock.  */
    if( newtime->tm_hour == 0 )        /*Set hour to 12 if midnight. *
+/
            newtime->tm_hour = 12;

    printf( "%.19s %s\n", asctime( newtime ), am_pm );    
    return 0; 
}
[download]

seems to do the appropriate thing,

P:\test>ltime TZ=PST8PDT
Thu Jan 19 12:20:55 AM
[download]

so it seems Perl is not doing so under Win32.

Theoretically, you could do the same steps shown in the C code above from within your perl script using Win32::API to import the CRT functions directly from the MS CRT DLL, but it is simpler to conditionally set the TZ and then exec yourself like this:

#! perl -slw
use strict;

unless( $ENV{TZ} ) {
    $ENV{TZ} = 'PST8PDT';
    exec "$^X $0";
}

print scalar localtime;

__END__
P:\test>524251

P:\test>Thu Jan 19 12:32:30 2006
[download]

but that's not so useful if you want to change the TZ many times. In that case it's probably easier to just spawn a perl one-liner and retrieve the values that way:

#! perl -slw
use strict;

print "default:" . localtime();

$ENV{TZ} = 'GST1GDT';
print `perl -le"print 'Germany: ' . localtime()"`;

$ENV{TZ} = 'PST8PDT';
print `perl -le"print 'Pacific: ' . localtime()"`;
__END__
P:\test>524251-2
default:Thu Jan 19 20:41:24 2006
Germany: Thu Jan 19 19:41:24 2006

Pacific: Thu Jan 19 12:41:24 2006
[download]