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

my $hashref =
{
    Inventory =>
    { 
        lb   => 'abc',
        host => [
                    { hostA => { os => 'Linux',   location => 'Dublin'  } },
                    { hostB => { os => 'Windows', location => 'US'      } },
                    { hostC => { os => 'Ubuntu',  location => 'Germany' } },
                ],
    },
};

# Question 1
my   @hosts = map { (keys %$_)[0] } @{ $hashref->{Inventory}{host} };
dd  \@hosts;

# Question 2
push @{ $hashref->{Inventory}{host} },
      { hostD => { os => 'Unix', location => 'France' } };
dd   $hashref;