Ah, after a bit of time, I realized the interplay here is more subtle than I was originally thinking.

My general rule is that, when loading a module conditionally, you have to work to avoid relying upon compile-time magic. The most obvious case of this would be to avoid code like:

if( $threads ) {
    require threads::shared; threads::shared->import();
    share @foo; # No parens used
}
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Because the "share @foo;" line would cause a compile-time error (if you did 'use strict;') even when that line would never be run.

An even more glaring case is the compile-time magic of the \& sub prototype that allows:

    use Exception::Spiffy 'catch';
    catch {
        ...
    };
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For conditional usage, you'd need to do it more like:

    require Exception::Spiffy; Exception::Spiffy->import('catch');
    &catch( sub {
        ...
    } );
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But there is a more subtle interaction with the \[$@%] prototype of threads::shared::share(). For example, this code will likely work:

if( $threads ) {
    require threads::shared; threads::shared->import();
    share( \@foo );
}
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But it also might not! If some module you loaded for whatever reason loaded something that loaded something that happened to load threads::shared, then that code would fail to compile, complaining:

Type of arg 1 to threads::shared::share must be one of [$@%] (not refe
+rence constructor)
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So, to be sure your code will work, you need to write it as:

if( $threads ) {
    require threads::shared; threads::shared->import();
    &share( \@foo ); # The ampersand is important
}
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