I solved the question, thank you all very much! The solution was

# i added 
use Encoding qw(encode decode);
# and in sub foo i decoded the things from File::Find. eg
sub foo {
    $file_name = decode("UTF-8", $_);
}
# cheers!
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here is the question and old code

# basically i want to go through a directory with File::Find
# and print the (non-asciian) filenames to a file.

# let's go through it incrementally, basic assumptions
use utf;
use File::Find;

open (OUT, '>:encoding(UTF-8)', '/tmp/tmp.sql');
open (IN, '<:encoding(ISO-8859-15)',$file_name);

sub foo {
    print "$_\n";
    print OUT "$_\n";
    # the IN file is used, but none of it's contents are
    # written to the out file, only the filename $_ is
    # needed in the OUT file
}

# when i now execute
find(\&foo, $mydir);
# it will print fine to the terminal (i use GNU/Debian)
# but mojibake to the file

# if i prepend this
binmode( STDOUT, ':encoding(UTF-8)'  ) or die $!;
binmode( STDIN, ':encoding(UTF-8)'  ) or die $!;
# to the script
# then it prints mojibake both to terminal and to file

# it would be more logical if the other way around
[download]

what is it that i miss to encode/decode?

I haven't had problems with utf files before, but this is the first time i use File::Find for interaction with the file system