Probably perlintro/perlsyn/perldata...

#!/usr/bin/perl --
use strict;
use warnings;
my %foo = ( 1, 2 , 1, 3, 1, 3, 1, 2 );
my %bar = ( %foo, 1, 3 );
use Data::Dumper;
print Dumper( \%foo, \%bar );
__END__
$VAR1 = {
          '1' => 2
        };
$VAR2 = {
          '1' => 3
        };
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