'perldoc perldiag'

Unsuccessful %s on filename containing newline

(W newline) A file operation was attempted on a filename, and that operation failed, PROBABLY because the filename contained a newline, PROBABLY because you forgot to chomp() it off. See chomp in the perlfunc manpage.

If you grep the perl source, you'll find these tests

########
# doio.c [Perl_my_stat Perl_my_lstat]

use warnings 'io' ;
stat "ab\ncd";
lstat "ab\ncd";
no warnings 'io' ;
stat "ab\ncd";
lstat "ab\ncd";
EXPECT
Unsuccessful stat on filename containing newline at - line 3.
Unsuccessful stat on filename containing newline at - line 4.
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########
# pp_sys.c [pp_stat]
use warnings 'newline' ;
stat "abc\ndef";
no warnings 'newline' ;
stat "abc\ndef";
EXPECT
Unsuccessful stat on filename containing newline at - line 3.
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So to disable that particular warning, through warnings, you simply write

    no warnings 'newline';
    next if -d "$dir/$current_filename";
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perllexwarn lists newline among others (though it doesn't really explain what warnings it blocks).