This error was reported under macOS 10.14.6.

My solution was:

1 remove Perl module GD (see https://www.perl.com/article/3/2013/3/27/How-to-cleanly-uninstall-a-Perl-module/)

2 Remove pkg-config

- brew uninstall pkg-config

3 Remove ImageMagick

- brew uninstall ImageMagick

4 Finally delete HomeBrew

- /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/uninstall.sh)"

This gave a me a "clean" system, to allow me to reinstall HomeBrew, pkg-config and finally add Perl module GD with cpan. I was unable to sucessfully reeinstall ImageMagick via HomeBrew, but had more luck using MacPorts.