$is_magick = 0;

if ( is_installed Image::Magick ) { $is_magick = 1 }

##</code><code>##

if ( $is_magick = 0 ) {

  print "Image::Magick functions disabled";
}
else {

  do stuff with Image::Magick ...
}

##</code><code>##

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;
}

##</code><code>##

our $is_magick = 1;
BEGIN {
  unless (eval "use Image::Magick") {
    $is_magick = 0;
  }
}