As always, TMTOWTDI.
I would probably use mplayer, as well. Something like

$mplayer_path = "/path/to/mplayer";
$video_file = "test_file.avi";

#create a temporary directory.
my $temppath = "/tmp/screenshots." . int(rand(1e7));

if( ! mkdir($temppath)){ print("Error: $!\n"); exit(1); }

if( ! chdir($temppath)){ print("Error: $!\n"); exit(1); }


# Pick a mode:
# Every single frame
# system($mplayer_path, "-vo", "png:z=3", "-ao", "null", $video_file);
# Every 20th frame
system($mplayer_path, "-vo", "png:z=3", "-vf", "framestep=20", "-ao", 
+"null", $video_file);
# Every keyframe
# system($mplayer_path, "-vo", "png:z=3", "-vf", "framestep=I", "-ao",
+ "null", $video_file);        

#mplayer creates many files of the form "^\d{8}\.png$" in the current 
+directory.

my $dirh;
if( ! opendir($dirh, $temppath)) { print("Error: $!\n"); exit(1); }
my @frames = readdir($dirh);
closedir($dirh);

foreach(@frames){
  #do something useful, or delete the file.
}
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