My approach has always been:

BEGIN {
    eval "use GD";
    $can_GD = 1 unless $@;
};
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Or something along those lines. Almost indistinguishable from the previous posts, admittedly, but I always try to be careful to wrap it up in BEGIN blocks. That way you're sure that the import is actually done at compile time instead of run time and it's virtually indistinguishable from an actual use.

So you don't need to worry about functions not be prototyped or whatnot.