To install Perl::Critic::Policy::Subroutines::ProhibitUnderscorePrivateSubs, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Perl::Critic::Policy::Subroutines::ProhibitUnderscorePrivateSubs
CPAN shell
perl -MCPAN -e shell
install Perl::Critic::Policy::Subroutines::ProhibitUnderscorePrivateSubs
For more information on module installation, please visit the detailed CPAN module installation guide.