The DateTime documentation implies that it respects $ENV{TZ}

No it doesn't. The documentation says

Determining the local time zone for a system can be slow. If $ENV{TZ} is not set, it may involve reading a number of files in /etc or elsewhere. If you know that the local time zone won't change while your code is running, and you need to make many objects for the local time zone, it is strongly recommended that you retrieve the local time zone once and cache it:

  our $App::LocalTZ = DateTime::TimeZone->new( name => 'local' );

  ... # then everywhere else

  my $dt = DateTime->new( ..., time_zone => $App::LocalTZ );
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So its explicit, use time_zone => 'local'

Well, I read this sentence:

If $ENV{TZ} is not set, it may involve reading a number of files in /etc or elsewhere.

to mean that DateTime checks $ENV{TZ}. Oh well.

$ENV{TZ} = "CEST";
tzset;
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Thanks, DateTime ignores it though.

So is explicit argument to constructor the only way to do it? Doesn't seem very Perlish to me :-)

So is explicit argument to constructor the only way to do it?

Yes, its the only way; you could create alternate constructor, or propose a patch for one

Doesn't seem very Perlish to me :-)

Why? time doesn't consult ENV{TZ} and neither does gmtime -- The module is called DateTime not, localtime :)