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    local $ENV{BLAH} = 42;
    libcall();
...
            print "env not set\n";
        }
    }

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

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