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

##</code><code>##

This is perl 5, version 12, subversion 2 (v5.12.2) built for i686-linux