Perl distinguishes between DB_file and DB_File. The module is named DB_File, so you better check again using:

perl -MDB_File -e1
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Also, I don't really understand your description of your installation. Did install DB_File via the CPAN tool or do:

perl Makefile.PL
make
make test
make install
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(the two are roughly equivalent) or did you do another form of "install"?