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;
}