use File::stat;
...

my %m;
@allPictures = sort {
  ($m{$a} ||= stat($a)->mtime) <=> ($m{$b} ||= stat($b)->mtime)
} @allPictures;

##</code><code>##

my %m;
@allPictures = sort {
  ($m{$a} ||= -M $a) <=> ($m{$b} ||= -M $b)
} @allPictures;