local $ENV{BLAH} = 42;
    libcall();

    sub libcall {
        if( exists $ENV{BLAH} ) {
            print "env set to $ENV{BLAH}\n";
        } else {
            print "env not set\n";
        }
    }

##</code><code>##

    if( condition() ) {
        local $ENV{BLAH} = 42;
    }
    libcall();

##</code><code>##

    if( condition() ) {
        local $ENV{BLAH} = 42;
        libcall();
    } else {
        libcall();
    }