A solution that doesn't use the second file, but builds the property table while reading the first file. This assumes that all required properties are present somewhere in the first file.

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{$_}}} k
+eys %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

[download]

Prints:

Computer1: KB896422 KB917159
Computer2: KB899588
Computer3: KB896423

[download]

Note: computer one's data was edited slightly from OP's data so that it misses two properties.

DWIM is Perl's answer to Gödel

In reply to Re: Finding out what computer does NOT have certain data by GrandFather
in thread Finding out what computer does NOT have certain data by Sunnmann

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