Here's a test script that illustrates one approach:

#!/usr/bin/perl -w

use strict;
use Data::Dumper;

my %hash;

print Dumper(\%hash);

$hash{one}->{key1} = 'value1';
$hash{one}->{key2} = 'value2';

print Dumper(\%hash);
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Note, I never actually assigned a value to $hash{one}. When I assigned a value to $hash{one}->{key1}, Perl automatically created the hashref for me before assigning 'value1' to the key 'key1'. This automatic creation of a data structure is called autovivification.

I like to use arrows to indicate dereferencing, but they are optional (and I think they're going away in Perl 6) so these two lines are equivalent:

$hash{one}->{key1} = 'value1';
$hash{one}{key1} = 'value1';
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