package PClass1;
use strict;
use Carp;

sub new {
    my $proto = shift;
    my $class = ref($proto) || $proto;
    my (%args) = @_;
    my $this = {
    _fs => $args{fs} || croak("You must supply a filesystem"),
    };
    bless $this, $class;
    return $this;
}


...

package PClass2;
use strict;

sub new {
    my $proto = shift;
    my $class = ref($proto) || $proto;
    my (%args) = @_;
    my $this = {
    _hosts => $args{hosts},
    };
    bless $this, $class;
    return $this;
}

......

package Subclass;
use strict;
use PClass1;
use PClass2;
our @ISA=qw(PClass1 PClass2);

sub new {
    my ($class, %args) = @_;
#################### Start of changes ####################
    my $obj1   = $class->PClass1::new(%args);
    my $obj2   = $class->PClass2::new(%args);
    $this = $class->PClass1::new(%args);

    my $this = {( %{$obj1}, %{$obj2} )};
    bless $this, $class;
#################### End of changes ####################
    return $this;
}