As it says those two lines are the contents of an eval done at line 114 from Text::CSV (which it appears you have a very slightly behind copy; in the latest that call has moved a few lines). The perldebug docs for f say that if you don't give it a full path the argument is taken as a regex which it searches for in the keys for %main::. If something else pulled in Text::CSV and it ran its eval and that happens to be the first match for qr{^_<.*CSV} as the keys happen to be ordered then that's what it'll switch to. Has nothing to do with @INC not being honoured.

Edit: the guts of the f command's search (5.34.0 version):

        # if not in magic file list, try a close match.
        if ( !defined $main::{ '_<' . $file } ) {
            if ( ($try) = grep( m#^_<.*$file#, keys %main:: ) ) {
                {
                    $try = substr( $try, 2 );
                    print $OUT "Choosing $try matching '$file':\n";
                    $file = $try;
                }
            } ## end if (($try) = grep(m#^_<.*$file#...
        } ## end if (!defined $main::{ ...
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