GD can do this (and more). Here's a quick 'n' dirty example that will create a 100x100 pixel PNG and spew it to STDOUT (so make sure you redirect to a file in your shell).

use strict;
use GD;

my $im = new GD::Image(100,100);
my @colors; # Preallocate colors, since PNG has a limited number of co
+lors
for (0..255) {
  $colors[$_] = $im->colorAllocate(int(rand(256)),int(rand(256)),int(r
+and(256)));
}

for my $x (0..99) {
    for my $y (0..99) {
        $im->setPixel($x,$y,$colors[int(rand(255))]);
    }
}
binmode STDOUT;
print $im->png;
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