#!/usr/bin/perl -l

package MyHash;
use Tie::Hash;
our @ISA = 'Tie::StdHash';

sub TIEHASH {
    my $class = shift;
    return bless {}, $class;
}

sub STORE {
    my $self = shift;
    print "storing  $_[0] => $_[1]";
    $self->SUPER::STORE(@_);
}

sub FETCH {
    my $self = shift;
    print "fetching @_";
    $self->SUPER::FETCH(@_);
}

sub CLEAR {
    my $self = shift;
    print "clear";
    $self->SUPER::CLEAR();
}


package main;
use strict;
use warnings;
use Data::Dumper;

tie my %myhash, "MyHash";

print "Variant 1 (bad):";

%myhash = ( %myhash, "key_$_" => { foo => 42 } )  for 1..5;

print "\nDumping...";
print Dumper \%myhash;

%myhash = ();

print "\nVariant 2 (good):";

$myhash{"key_$_"} = { foo => 42 }  for 1..5;

print "\nDumping...";
print Dumper \%myhash;

__END__
Variant 1 (bad):
clear
storing  key_1 => HASH(0x65ecb0)
fetching key_1
clear
storing  key_1 => HASH(0x65ecb0)
storing  key_2 => HASH(0x6c2050)
fetching key_1
fetching key_2
clear
storing  key_1 => HASH(0x65ecb0)
storing  key_2 => HASH(0x6c2050)
storing  key_3 => HASH(0x74a420)
fetching key_3
fetching key_1
fetching key_2
clear
storing  key_3 => HASH(0x74a420)
storing  key_1 => HASH(0x65ecb0)
storing  key_2 => HASH(0x6c2050)
storing  key_4 => HASH(0x74a3f0)
fetching key_3
fetching key_1
fetching key_4
fetching key_2
clear
storing  key_3 => HASH(0x74a420)
storing  key_1 => HASH(0x65ecb0)
storing  key_4 => HASH(0x74a3f0)
storing  key_2 => HASH(0x6c2050)
storing  key_5 => HASH(0x74aaf0)

Dumping...
fetching key_3
fetching key_1
fetching key_5
fetching key_2
fetching key_4
$VAR1 = {
          'key_3' => {
                       'foo' => 42
                     },
          'key_1' => {
                       'foo' => 42
                     },
          'key_5' => {
                       'foo' => 42
                     },
          'key_2' => {
                       'foo' => 42
                     },
          'key_4' => {
                       'foo' => 42
                     }
        };

clear

Variant 2 (good):
storing  key_1 => HASH(0x74ac80)
storing  key_2 => HASH(0x737c40)
storing  key_3 => HASH(0x6c2050)
storing  key_4 => HASH(0x74ad00)
storing  key_5 => HASH(0x74ac00)

Dumping...
fetching key_3
fetching key_1
fetching key_5
fetching key_4
fetching key_2
$VAR1 = {
          'key_3' => {
                       'foo' => 42
                     },
          'key_1' => {
                       'foo' => 42
                     },
          'key_5' => {
                       'foo' => 42
                     },
          'key_4' => {
                       'foo' => 42
                     },
          'key_2' => {
                       'foo' => 42
                     }
        };