# How many pictures you have in your rotate folder
my $num_of_pics = 10;
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Or you could get the actual number of pictures:

# How many pictures you have in your rotate folder
my $num_of_pics = () = <~/Pictures/rotate/*.jpg>;
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Or just use ordinarily named files and pic one at random:

my @all_pics = <~/Pictures/rotate/*.jpg>;

print `feh --bg-center $all_pics[ rand @all_pics ]`;
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# Get a random number from your total of pictures
my $pic = int(rand($num_of_pics)+1); 
chomp($pic);
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There is no need to chomp a number because there is no newline at the end of it.