/usr/lib/x86_64-linux-gnu/libfftw3l.so.3 is in libfftw3-long3 3.3.5-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libfftw3l.so.3.5.5
/usr/lib/x86_64-linux-gnu/libfftw3l.so.3.5.5