The step "carton install --deployment" in the Perl dependencies section of INSTALL requires libicu-dev in order to install Unicode::ICU::Collator, otherwise you get

Configuring Unicode-ICU-Collator-0.002
Running Makefile.PL
OS unsupported: No icu-config --cppflags found
-> N/A
-> FAIL Configure failed for Unicode-ICU-Collator-0.002. See /home/nikki/.cpanm/build.log for details.

libicu-dev is mentioned later in the section about PostgreSQL extensions, but it's a bit late by then. It's a dependency for the Perl modules so should be mentioned in the Perl dependencies section.