diff --git a/src/lib/Hydra/Plugin/BitBucketStatus.pm b/src/lib/Hydra/Plugin/BitBucketStatus.pm
index 72ed3b6b..33e3bc03 100644
--- a/src/lib/Hydra/Plugin/BitBucketStatus.pm
+++ b/src/lib/Hydra/Plugin/BitBucketStatus.pm
@@ -10,7 +10,7 @@ use Hydra::Helper::CatalystUtils;
 
 sub isEnabled {
     my ($self) = @_;
-    return $self->{config}->{enable_bitbucket_status} == 1;
+    return ($self->{config}->{enable_bitbucket_status} // 0) == 1;
 }
 
 sub toBitBucketState {
diff --git a/src/lib/Hydra/Plugin/CoverityScan.pm b/src/lib/Hydra/Plugin/CoverityScan.pm
index c7b39d02..0a7fae0c 100644
--- a/src/lib/Hydra/Plugin/CoverityScan.pm
+++ b/src/lib/Hydra/Plugin/CoverityScan.pm
@@ -53,7 +53,7 @@ sub buildFinished {
     my $covTarball;
 
     opendir my $tarballs_handle, $tarballs or die;
-    while (my $file = readdir $tarballshandle) {
+    while (my $file = readdir $tarballs_handle) {
         next unless $file =~ /.*-coverity-int\.(tgz|lzma|xz|bz2|zip)$/;
         $covTarball = "$tarballs/$file"; last;
     }
diff --git a/src/lib/Hydra/Plugin/EmailNotification.pm b/src/lib/Hydra/Plugin/EmailNotification.pm
index 721365e1..b5f133a6 100644
--- a/src/lib/Hydra/Plugin/EmailNotification.pm
+++ b/src/lib/Hydra/Plugin/EmailNotification.pm
@@ -12,7 +12,7 @@ use Hydra::Helper::Email;
 
 sub isEnabled {
     my ($self) = @_;
-    return $self->{config}->{email_notification} == 1;
+    return ($self->{config}->{email_notification} // 0) == 1;
 }
 
 my $template = <<EOF;
diff --git a/src/lib/Hydra/Plugin/GitInput.pm b/src/lib/Hydra/Plugin/GitInput.pm
index cd8d23e2..39a5cf83 100644
--- a/src/lib/Hydra/Plugin/GitInput.pm
+++ b/src/lib/Hydra/Plugin/GitInput.pm
@@ -34,7 +34,7 @@ sub _parseValue {
     my $start_options = 3;
     # if deepClone has "=" then is considered an option
     # and not the enabling of deepClone
-    if (index($deepClone, "=") != -1) {
+    if (defined($deepClone) && index($deepClone, "=") != -1) {
         undef $deepClone;
         $start_options = 2;
     }
diff --git a/src/lib/Hydra/Plugin/GithubRefs.pm b/src/lib/Hydra/Plugin/GithubRefs.pm
index 1de2672d..2038ea47 100644
--- a/src/lib/Hydra/Plugin/GithubRefs.pm
+++ b/src/lib/Hydra/Plugin/GithubRefs.pm
@@ -97,8 +97,8 @@ sub _iterate {
 }
 
 sub fetchInput {
-    my ($self, $type, $name, $value, $project, $jobset) = @_;
-    return undef if $type ne "github_refs";
+    my ($self, $input_type, $name, $value, $project, $jobset) = @_;
+    return undef if $input_type ne "github_refs";
 
     my ($owner, $repo, $type, $fut, $prefix) = split ' ', $value;
     die "type field is neither 'heads' nor 'tags', but '$type'"
diff --git a/src/lib/Hydra/Plugin/S3Backup.pm b/src/lib/Hydra/Plugin/S3Backup.pm
index bdd23b2f..a67082ec 100644
--- a/src/lib/Hydra/Plugin/S3Backup.pm
+++ b/src/lib/Hydra/Plugin/S3Backup.pm
@@ -21,11 +21,18 @@ sub isEnabled {
 }
 
 my $client;
-my %compressors = (
-    xz => "| $Nix::Config::xz",
-    bzip2 => "| $Nix::Config::bzip2",
-    none => ""
-);
+my %compressors = ();
+
+$compressors{"none"} = "";
+
+if (defined($Nix::Config::bzip2)) {
+    $compressors{"bzip2"} = "| $Nix::Config::bzip2",
+}
+
+if (defined($Nix::Config::xz)) {
+    $compressors{"xz"} = "| $Nix::Config::xz",
+}
+
 my $lockfile = Hydra::Model::DB::getHydraPath . "/.hydra-s3backup.lock";
 
 sub buildFinished {
diff --git a/src/script/hydra-eval-jobset b/src/script/hydra-eval-jobset
index 4a165d86..7a9af09f 100755
--- a/src/script/hydra-eval-jobset
+++ b/src/script/hydra-eval-jobset
@@ -567,7 +567,7 @@ sub checkJobsetWrapped {
     my $inputInfo = {};
     if ($jobsetsJobset) {
         my @declInputs = fetchInput($plugins, $db, $project, $jobset, "decl", $project->decltype, $project->declvalue, 0);
-        my $declInput = @declInputs[0] or die "cannot find the input containing the declarative project specification\n";
+        my $declInput = $declInputs[0] or die "cannot find the input containing the declarative project specification\n";
         die "multiple alternatives for the input containing the declarative project specification are not supported\n"
             if scalar @declInputs != 1;
         my $declFile = $declInput->{storePath} . "/" . $project->declfile;