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!
}
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.