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my %lookup;
@lookup{@in} = ();
...
    @db = grep {not exists $lookup{$_}} @db;
  }
}

my (%db_lookup, %in_lookup);
@db_lookup{@db} = ();
...
} elsif ($condition2) {
  push @db, grep { not exists $db_lookup{$_} } @in;
}