Brew upgrade freezes on [email protected] make test

[mmuftwork]

For a few days now, brew upgrade has failed for [email protected]. Each time it gets to the make test stage and stops. I've left it running over 24 hours so it isn't an issue of I didn't wait long enough.

The terminal window where it is running displays the message: perl * ruby -WO --disable=gems,did_you_mean,rubyopt /user/local/Homebrew/brew.rb upgrade on its head.

I'm running High Sierra 10.13.6. All the formulae and casks were quite up to date until this.