You can use ImageMagick's compare. See PerlMagick page and search for Compare to do it in Perl. In shell, it would go something like

 compare -metric mae image.png reference.png difference.png
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in Perl, I havn't figured out the right syntax. If you do, please post it.

Update: I found this on Google. The shell compare returns an image with red pixels where the diff's are. Apparently, Compare in PerlMagick just returns some statistics like the following returns:

Errors is 3156.326700
Mean Error is 0.026116
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#!/usr/bin/perl
use warnings;
use strict;
use Image::Magick;

my $i1 = Image::Magick->new;
my $i2 = Image::Magick->new;

my $w = $i1->Read('image.png'); # read in images
warn("$w")  if $w;
exit  if $w =~ /^Exception/;

$w = $i2->Read( 'reference.png');
warn("$w")  if $w;
exit  if $w =~ /^Exception/;

$i1->Scale(width=>100, height=>100); # scale without preserving aspect
+ ratio
$i2->Scale(width=>100, height=>100);

$w = $i1->Compare(image=>$i2);       # compare
die "$w" if $w;

printf "Errors is %f\n",     $i1->Get('error');
printf "Mean Error is %f\n", $i1->Get('mean-error');
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UPDATE2: Found a way with the Difference option of Composite, to produce a visual diff.

#!/usr/bin/perl
use warnings;
use strict;
use Image::Magick;

my $i1 = Image::Magick->new;
my $i2 = Image::Magick->new;

#make some test files
$i1->Set(size=>'250x100');
$i1->ReadImage('xc:white');
$i1->Draw(primitive=> 'rectangle',
          fill=>'red', 
          points=>'60,30 110,70');
$i1->Write("i1.png");  #for comparison

$i2->Set(size=>'250x100');
$i2->ReadImage('xc:white');
$i2->Draw(primitive=> 'line',
          fill=>'green',
          points=>'10,10 200,100');
$i2->Write("i2.png");  #for comparison

$i1->Composite(
     gravity => "Center", 
     compose=>'Difference',
     image => $i2,
     );
$i1->Write("$0-diff-compose.png");
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