I've just run my build-order script for the first time after the 5.18 rebuild, and came up with some build dependency cycles, such as:

perl-Perl-OSType->perl-Capture-Tiny->perl-Inline->perl-Parse-RecDescent->perl-Module-Build->perl-Perl-OSType

perl-Test-Pod->perl-Module-Build->perl-Perl-OSType->perl-Capture-Tiny->perl-Inline->perl-Parse-RecDescent->perl-Test-Pod

perl-Perl-OSType->perl-Capture-Tiny->perl-Parse-RecDescent->perl-Module-Build->perl-Perl-OSType

perl-Test-Pod->perl-Module-Build->perl-Perl-OSType->perl-Capture-Tiny->perl-Parse-RecDescent->perl-Test-Pod

perl-Test-Warn->perl-Test-Pod->perl-Module-Build->perl-Perl-OSType->perl-Capture-Tiny->perl-Inline->perl-Parse-RecDescent->perl-Test-Warn

perl-Tree-DAG_Node->perl-Test-Pod->perl-Module-Build->perl-Perl-OSType->perl-Capture-Tiny->perl-Inline->perl-Parse-RecDescent->perl-Test-Warn->perl-Tree-DAG_Node

perl-Test-Warn->perl-Test-Pod->perl-Module-Build->perl-Perl-OSType->perl-Capture-Tiny->perl-Inline->perl-Test-Warn

perl-Tree-DAG_Node->perl-Test-Pod->perl-Module-Build->perl-Perl-OSType->perl-Capture-Tiny->perl-Inline->perl-Test-Warn->perl-Tree-DAG_Node

perl-Test-Warn->perl-Test-Pod->perl-Module-Build->perl-Perl-OSType->perl-Capture-Tiny->perl-Parse-RecDescent->perl-Test-Warn

perl-Tree-DAG_Node->perl-Test-Pod->perl-Module-Build->perl-Perl-OSType->perl-Capture-Tiny->perl-Parse-RecDescent->perl-Test-Warn->perl-Tree-DAG_Node

perl-Test-Warn->perl-Sub-Uplevel->perl-Test-Script->perl-IPC-Run3->perl-Test-Pod->perl-Module-Build->perl-Perl-OSType->perl-Capture-Tiny->perl-Inline->perl-Parse-RecDescent->perl-Test-Warn

perl-Test-Warn->perl-Sub-Uplevel->perl-Test-Script->perl-IPC-Run3->perl-Test-Pod->perl-Module-Build->perl-Perl-OSType->perl-Capture-Tiny->perl-Inline->perl-Test-Warn

perl-Test-Warn->perl-Sub-Uplevel->perl-Test-Script->perl-IPC-Run3->perl-Test-Pod->perl-Module-Build->perl-Perl-OSType->perl-Capture-Tiny->perl-Parse-RecDescent->perl-Test-Warn

All of these would go away if perl-Capture-Tiny didn't include the optional test buildreqs Inline/Inline::C/Parse::RecDescent when bootstrapping.

It might be argued that all of these cycles involve Module::Build, which, being a dual-lived package, would be available at bootstrap time from the main bootstrapped perl package, but I'd prefer to avoid making special cases for such things where possible.