use strict ; use warnings ;

my %hash = () ;

$hash{'dummy'} = undef ;
printf STDERR "Single entry requires: %d Bytes\n", bytes($hash{dummy}) ;

read_it() ;

undef %hash ;
show(0, \%hash) ;

wait_for_it('Just "undef"ed the hash') ;

read_it() ;

sub read_it {
  open my $FH, "<", "hash.txt" ;

  wait_for_it('About to read') ;

  my $e = 0 ;
  my $c = show($e, \%hash) ;

  while (<$FH>) {
    $hash{$_} = undef ;
    $e++ ;
    $c = ($c - 1) || show($e, \%hash) ;
  } ;

  wait_for_it('Finished Reading') ;
} ;

sub show {
  my ($e, $rh) = @_ ;
  printf STDERR "%8d entries: %3.1fM Bytes\n", $e, mbytes($rh) ;
  return 50_000 ;
} ;

sub mbytes {
  my ($r) = @_ ;
  bytes($r)/(1024 * 1024) ;
} ;

use Devel::Size () ;

sub bytes {
  my ($r) = @_ ;
  return Devel::Size::size($r) ;
} ;

sub wait_for_it {
  print STDERR "$_[0]..." ;
  my $ch = '' ;
  while ($ch !~ m/\./) { sysread STDIN, $ch, 1 ; } ;
} ;