use strict;
use warnings;

my %properties;
my @computers;

while (<DATA>) {
    chomp;
    next if ! length;
    
    push @computers, $_;
    
    while (<DATA>) {
        chomp;
        last if ! length;
        push @{$properties{$_}}, $computers[-1];
    }
}

for my $computer (@computers) {
    my @missing = grep {! grep {$_ eq $computer} @{$properties{$_}}} keys %properties;
    
    $computer = [$computer, [@missing]];
}

print join "\n", map {"$_->[0]: @{$_->[1]}"} @computers;

__DATA__
Computer1
KB893756
KB896423
KB899588
KB899591
KB921883

Computer2
KB893756
KB896422
KB896423
KB899591
KB917159
KB921883

Computer3
KB893756
KB896422
KB899588
KB899591
KB917159
KB921883

##</code><code>##

Computer1: KB896422 KB917159
Computer2: KB899588
Computer3: KB896423