Apparently it may not always work, as I learned when I researched this further (see RE: Checking to see if a particular Module is installed). merlyn's response was to use the following:

eval 'require GD';
if ($@) {
  # problems with GD, fall back to non-GD
} else {
  # we have GD, go for it!
}
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although there appears to be some caveats to that approach.

If you are open to using other modules to do the check, you could look into ExtUtils::Installed and Module::Load::Conditional.

See also a recent thread on this topic: Check if module is installed.