Subclass File::Temp::Dir and override the DESTROY method. This is somewhat complicated by the fact that File::Temp::Dir doesn't have a traditional constructor.

package My::File::Temp::Dir;
use File::Temp ();
our @ISA = qw(File::Temp::Dir);
sub new {
  my $class = shift;
  my $self  = File::Temp->newdir(@_);
  bless $self, $class;
}
sub DESTROY {
  my $tempdir = shift;
  ...;  # do stuff
  $tempdir->SUPER::DESTROY(@_);
}
1;
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Then, stop using the tempdir function and use My::File::Temp::Dir->new instead.