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use File::stat;
...
...
@allPictures = sort {
  ($m{$a} ||= stat($a)->mtime) <=> ($m{$b} ||= stat($b)->mtime)
} @allPictures;

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