Check out Image::Info. That module handles lots of image formats and provides a dim() function. Use it in list context to get both X and Y dimensions and then multiply for the number of pixels. This code is untested, but it should be as easy as:

use Image::Info; 
my $info = image_info($image_file);
my ($x,$y) = dim($info);
print "Total pixels: ", $x * $y, "\n";
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-sauoq
"My two cents aren't worth a dime.";