Searching CPAN is often a good place to start; for your specific problem the CPAN Graphics section would be a good choice.

I know the GD module has these methods:

• ($width,$height) = $image->getBounds()
• $width = $image->width
• $height = $image->height
• $flag = $image1->compare($image2)

There may be better choices depending on your application. For instance, if you need to do this within a GUI, Tk::Photo might be more appropriate.