Consider:

#! perl
use strict;
use warnings;
use Data::Dump;

my %hash1 = ( fred   => [ 'wilma', 'pebbles'   ],
              barney => [ 'betty', 'bamm-bamm' ], );
my %hash2 = ( homer  => [ 'marge', 'bart', 'lisa', 'maggie' ],
              romeo  => [ 'juliet' ] );
my $a     =  'fred';
my $b     =  'homer';

print "Before the assignments:\n";
print "hash1: ";
dd \%hash1;
print "hash2: ";
dd \%hash2;

my $sub = $hash1{$a} = $hash2{$b};

print "\nAfter the assignments:\n";
print "hash1: ";
dd \%hash1;
print "hash2: ";
dd \%hash2;
print "sub:   ";
dd  $sub;

push @$sub, $a;

print "\nAfter the push:\n";
print "sub:   ";
dd  $sub;
[download]

Output:

12:39 >perl 765_SoPW.pl
Before the assignments:
hash1: { barney => ["betty", "bamm-bamm"], fred => ["wilma", "pebbles"
+] }
hash2: { homer => ["marge", "bart", "lisa", "maggie"], romeo => ["juli
+et"] }

After the assignments:
hash1: {
  barney => ["betty", "bamm-bamm"],
  fred   => ["marge", "bart", "lisa", "maggie"],
}
hash2: { homer => ["marge", "bart", "lisa", "maggie"], romeo => ["juli
+et"] }
sub:   ["marge", "bart", "lisa", "maggie"]

After the push:
sub:   ["marge", "bart", "lisa", "maggie", "fred"]

12:39 >
[download]

First, the %hash2 value with key $b is assigned to the %hash1 value with key $a.

Next, this value (which in my example code is an array reference) is assigned to $sub.

Finally, $a is pushed onto the array referenced by $sub.

Hope that helps,