I can't figure out where to install the library for ActivePerl to use it

Assuming you're using a Microsoft Compiler with ActivePerl, the header file(s) (.h) can go anywhere, but their location needs to be in the INCLUDE environment variable. Add that location if need be:

set INCLUDE=%INCLUDE%;C:\my_tiff_headers
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The import library (.lib) or static library (.lib) can go anywhere, but its location needs to be in the LIB environment variable. Add that location if need be:

set LIB=%LIB%;C:\my_tiff_lib
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And if it's a dynamic (not static) library, the dll can go anywhere, but its location needs to be in the PATH environment variable. Add that location if need be:

set PATH=%PATH%;C:\my_tiff_dll
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That done, the files are all in locations where they can be found.

If you're using the MinGW compiler, the principle is the same, but the details differ. (The import/static library has a ".a" extension, the CPATH environment variable is used instead of INCLUDE, and the LIBRARY_PATH environment variable is used instead of LIB.)

Probably easier to just install the ppm package :-)

Cheers, Rob