Localising the $ENV{TZ} isn't sufficient to prevent the effect being global, tzset() will need to be called after the value has been reverted to restore the timezone used by localtime(). For example:

use POSIX qw(tzset);
use strict;
                                                                      
+          
print scalar localtime();
                                                                      
+          
sub localtime_in
{
   local $ENV{TZ} = 'EST';
                                                                      
+          
   tzset();
                                                                      
+          
   localtime();
}
                                                                      
+          
print scalar localtime_in();
print scalar localtime();
[download]

Off the top of my head a way of fixing that would be something like:

use POSIX qw(tzset);
use strict;
                                                                      
+          
print scalar localtime();
                                                                      
+          
sub localtime_in
{
   my $rc;
   my @rc;
   {
     local $ENV{TZ} = shift;
     tzset();
     wantarray ? @rc = localtime() : $rc = localtime();
   }
   tzset();
   return wantarray ? @rc : $rc;
}
                                                                      
+          
print scalar localtime_in('EST');
print scalar localtime();
[download]

/J\