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use File::PathConvert qw(realpath $resolved);

sub work_with_image {
  my ($image_file) = @_; # untrusted
  $image_file = realpath("/img/$image_file") || croak "Resolution stopped at $resolved";
  if ($image_file !~ m|/img/|) {
    croak "Image file not in /img";
  }
  open( my $image, "<", $image_file) or croak "Can't open $image_file";
  # ...
}

use Cwd 'abs_path';

sub work_with_image {
  my ($image_file) = @_; # untrusted
  $image_file = abs_path("/img/$image_file");
  $filename = abs_path( $filename);
  if ($image_file !~ m|/img/|) {
    croak "Image file not in /img";
  }
  open( my $image, "<", $image_file) or croak "Can't open $image_file";
  # ...
}