No, but how can one assign to a hash element that does not exist? Well, %ENV is magical, so let's test it out.

$ perl -wE'
   my $condition = 0;
   say exists($ENV{BLAH}) || 0;
   local $ENV{BLAH} = $condition ? 42 : $ENV{BLAH};
   say exists($ENV{BLAH}) || 0;
'
0
1
[download]

So I was right

This is perl 5, version 12, subversion 2 (v5.12.2) built for i686-linu
+x
[download]

My tests appeared to work for strange reasons. Sorry about that. But you can fix the original without copying the whole environment:

    local $ENV{BLAH};
    if(  @ARGV  ) {
        $ENV{BLAH}= 42;
    } else {
        delete $ENV{BLAH};
    }
[download]

- tye