opendir my $d, $dir or die "Can't readdir $dir: $!";
my @files =
  grep -f $_,                        # collect only the files in folder
  map "$dir/$_", readdir $d;         # make full pathnames to the files
closedir $d;  # close directory handle

# sort using a "Schwartzian Transform"
my @sorted =
  map $_->[ 0 ],                     # untag
  sort { $b->[ 1 ] <=> $a->[ 1 ] }   # sort according to age
  map [ $_, -M $_ ],                 # tag files with their "ages"
  @files;

unlink @sorted[ 0, 1 ] if @sorted > 20;