X = Y = Z

##</code><code>##

Y = Z
X = Y

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

use Data::Dumper;

{
    # Contrived, arbitrary values for your variables
    my %hash1;
    my %hash2 = (b => [ 1, 2, 3 ]);
    my ($a, $b) = qw{a b};

    # Your code
    my $sub = $hash1{$a} = $hash2{$b};
    push @$sub, $a;

    # Your data
    print Dumper $sub;
}

{
    # Possibly more meaningful variable names (with same values)
    my %some_HoA;
    my %other_HoA = (b => [ 1, 2, 3 ]);
    my ($key_a, $key_b) = qw{a b};

    # Possibly more understandable code
    $some_HoA{$key_a} = $other_HoA{$key_b};
    my $some_array_ref = $some_HoA{$key_a};
    push @$some_array_ref, $key_a;

    # More obvious data?
    print Dumper $some_array_ref;
}

##</code><code>##

$VAR1 = [
          1,
          2,
          3,
          'a'
        ];
$VAR1 = [
          1,
          2,
          3,
          'a'
        ];