This will do the job, including comparing different image formats provided they contain the same picture:

#! perl -slw
use strict;
use GD;

GD::Image->trueColor( 1 );

my $img1 = GD::Image->new( $ARGV[ 0 ] );
my $img2 = GD::Image->new( $ARGV[ 1 ] );

die "Different dimensions"
    unless $img1->width  == $img2->width
    and    $img1->height == $img2->height;

my $raw1 = $img1->gd;
my $raw2 = $img2->gd;

die "images are different"
    unless $raw1 eq $raw2;

print "Images are the same";

__END__
c:\test>cmpImages mandrill.jpg mandrill.png
Images are the same
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But be aware, that images that look the same to the our eyes may be quite different in detail.