carton update [module]
Carton is designed to update your dependency in a conservative way, meaning that it doesn't update modules that aren't explicitly required to.
"carton update" is a command to explicitly update one or all of modules in your cpanfile to the latest available that satisfies the requirements in cpanfile.
requires 'DBI', '1.600'; requires 'Plack', '== 1.0011';
and then run "carton install" to get DBI 1.610 (the latest at that time) and Plack 1.0011 (as specified in the requirement).
A few weeks later, DBI and Plack have been updated a couple of times. Running "carton install" won't update the versions, because the installed versions satisfy the requirements in "cpanfile".
Running "carton update" will update DBI to the latest version, say 1.611, because the version still satisfies the requirement. However, it won't update Plack's version, since whatever latest version on CPAN will not satisfy the Plack's requirement "== 1.0011" because it wants an exact version.