my %lookup;
@lookup{@in} = ();
# For every $elem in @db ... 
foreach my $elem (@db) {
  # see if it exists in the lookup hash ...
  if (exists $lookup{$elem}) { 
    # and if it does then search @db (*again*) looking for
    # items that do not appear in the lookup hash. You're
    # also modifying the array you're iterating over (@db),
    # which can yield unpredictable results, so don't do it ;)
    @db = grep {not exists $lookup{$_}} @db;
  }
}

##</code><code>##

my (%db_lookup, %in_lookup);
@db_lookup{@db} = ();
@in_lookup{@in} = ();

if ($condition1) { # Dunno what your test conditions are, so...
  @db = grep { not exists $in_lookup{$_} } @db;
} elsif ($condition2) {
  push @db, grep { not exists $db_lookup{$_} } @in;
}