Another option is to use List::Compare :
Untested pseudocode ..

use List::Compare;

my @required_patches= qw(patch numbers that are required);
my @current_machine_patches= ....;

my $lc =  List::Compare->new(
  {lists => [\@required_patches, \@current_machine_patches],
        accelerated => 1,
        unsorted => 1
  });

my @missing_patches = $lc->get_Lonly;
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