You never show where exactly in a regex you insert your look-behind, so I can only get what you're doing wrong.

If you put it at the start of a regex, you need to a negative look-ahead, because the IP address is not yet matched. Example:

use strict;
use warnings;
use 5.010;

my $localhost = qr/(?!10\.20\.11\.99\s)/;

while (<DATA>) {
    next if /^\s*$/;
    next if /^\s*#.*$/;
    if (/^$localhost([a-fA-F0-9:\.]+)(\s+[a-zA-Z0-9\.-]+)+\s*$/) {
        say "IP: $1; HOST: $2";
    }
    else {
        chomp;
        say "No match for '$_'";
    }
}


__DATA__
127.0.0.1       localhost
10.20.11.99     kvm-test-v06.example.com        kvm-test-v06

# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

# Spurious records
1.2.3.4 litter trash kvm-test-v06.example.com   garbage
1.2.3.5 kvm-test-v06 more garbage
1.2.3.6 litter kvm-test-v06
1.2.4.7 kvm-test-v06.example.com kvm-test-v06
[download]

Produces the output:

IP: 127.0.0.1; HOST:    localhost
No match for '10.20.11.99       kvm-test-v06.example.com        kvm-te
+st-v06'
IP: ::1; HOST:  ip6-loopback
IP: fe00::0; HOST:  ip6-localnet
IP: ff00::0; HOST:  ip6-mcastprefix
IP: ff02::1; HOST:  ip6-allnodes
IP: ff02::2; HOST:  ip6-allrouters
IP: 1.2.3.4; HOST:      garbage
IP: 1.2.3.5; HOST:  garbage
IP: 1.2.3.6; HOST:  kvm-test-v06
IP: 1.2.4.7; HOST:  kvm-test-v06
[download]

Note that this uses the negative look-ahead directly after the ^ anchor, so that the look-ahead never backtracks.

I would like to delete all those records which contain the host name (qualified or unqualified), but don't start with the right address of the machine

It is much easier to do such a check after you have parsed the line, and extracted host name and IP address.