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

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

and all is well, because the code within libcall() now sees $ENV{BLAH}'s value, while other parts of the system (outside the scope of the snippet above) aren't affected by the setting.

But now I want to add a condition in my code, to perform the setting only when another condition is met, but I can't use

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

because the 'local' directive ends with the scope of the if-body and libcall() won't see it. I can't set the variable to undef because the library checks if the value exists(). I could obviously do

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

but that's lame because of the code duplication.

Is there a better way to 'local'ize a variable conditionally?