Some ports, particularly the p5- ports, need to change their pkg-plist depending on what options they are configured with (or version of perl, in the case of p5- ports). To make this easy, any instances in the pkg-plist of %%OSREL%%, %%PERL_VER%%, and %%PERL_VERSION%% will be substituted for appropriately. The value of %%OSREL%% is the numeric revision of the operating system (e.g., 4.9). %%PERL_VERSION%% is the full version number of perl (e.g., 5.00502) and %%PERL_VER%% is the perl version number minus the patchlevel (e.g., 5.005). Several other %%VARS%% related to port's documentation files are described in the relevant section.
If you need to make other substitutions, you can set the PLIST_SUB variable with a list of VAR=VALUE pairs and instances of %%VAR%% will be substituted with VALUE in the pkg-plist.
For instance, if you have a port that installs many files in a version-specific subdirectory, you can put something like
OCTAVE_VERSION= 2.0.13
PLIST_SUB= OCTAVE_VERSION=${OCTAVE_VERSION}
in the Makefile and use %%OCTAVE_VERSION%% wherever the version shows up in pkg-plist. That way, when you upgrade the port, you will not have to change dozens (or in some cases, hundreds) of lines in the pkg-plist.
This substitution (as well as addition of any manual pages) will be done between the pre-install and do-install targets, by reading from PLIST and writing to TMPPLIST (default: WRKDIR/.PLIST.mktmp). So if your port builds PLIST on the fly, do so in or before pre-install. Also, if your port needs to edit the resulting file, do so in post-install to a file named TMPPLIST.
Another possibility to modify port's packing list is based on setting the variables PLIST_FILES and PLIST_DIRS. The value of each variable is regarded as a list of pathnames to write to TMPPLIST along with PLIST contents. Names listed in PLIST_FILES and PLIST_DIRS are subject to %%VAR%% substitution, as described above. Except for that, names from PLIST_FILES will appear in the final packing list unchanged, while @dirrm will be prepended to names from PLIST_DIRS. To take effect, PLIST_FILES and PLIST_DIRS must be set before TMPPLIST is written, i.e. in pre-install or earlier.