Your first snippet of: 
our $is_magick = 0;
eval 'require GD';
if ($@) {
  # problems with GD, fall back to non-GD
} else {
  # we have GD, go for it!
  $is_magick = 1;
}

[download]
works exactly as you want, when run as written above. $is_magick is set to 0 if GD is not available and 1 if GD is available.

That suggests that there's something about your implementation of this snippet in your module or program that prevents it from working.

Can you provide a sample of code that you have written which demonstrates erroneous behaviour?

In reply to Re: disable functions if module not installed by Anonymous Monk
in thread disable functions if module not installed by lepetitalbert

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