...
my %installed;  # these will be HoA
my %hotfix;     # where the hash keys are
my %ready;      # taken from @valid_os

my $ninstalled = 0;

foreach my $os (@valid_os) {
    my $path_os = $PATCHPATH  . $os;
    opendir( DIR, $path_os ) or die "dir $path_os not found: $!";
    while (my $_ = readdir(DIR)) {
        next if ( /^\.\.?$/ );
        push( @{$installed{$os}}, $_ );  ### SEE NOTE BELOW
        $ninstalled++;
    }
    closedir DIR;
}

die "No hotfix files are installed in $PATCH_PATH\n" unless ($ninstalled);

...
while ($db->FetchRow) {
    my %hotfix_record = $db->DataHash("os","swname");
    push( @{$hotfix{$os}}, $hotfix_record{swname} );
}
...

for my $os (@valid_os) {
    @{$ready{$os}} = Check_Array( @{$installed{$os}}, @{$hotfix{$os}} );

    for my $hotfix ( @{$ready{$os}} ) {
        Add_Hotfix( $hotfix, $os );
    }
}