sub load {
    my ($qfn) = @_;
    open(my $fh, '<', $qfn)
        or die("Can't open file \"$qfn\": $!\n");
    
    my %data;
    local *_;
    while (<$fh>) {
        my ($id) = /^(\S+)/ or next;
        die("Duplicate key $id\n")
            if exists($data{$id});
        $data{$id} = $_;
    }

    return \%data;
}

my $master = eval { load(...) }
    or die("Can't load master db: $@");

my $new = eval { load(...) }
    or die("Can't load new db: $@");

my %ids;
for my $id (keys(%$master), keys(%$new)) {
    next if $ids{$id}++;
    if (!exists($new->{$id})) {
        print("-$master->{$id}");
    }
    elsif (!exists($master->{$id})) {
        print("+$new->{$id}");
    }
    elsif ($master->{$id} ne $new->{$id}) {
        print("-$master->{$id}");
        print("+$new->{$id}");
    }
    else {
        print("=$master->{$id}");
    }
}
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Update: Added missing parens and fixed var name as per reply.