I've just tried:

use utf8;

ex("c:\\fil\x{e9}.txt");
ex("c:\\fil" . pack("U", 0xe9 ) .  ".txt");

sub ex {
    my $f2 = shift;
    my $f = $f2;
    print $f;
    print ((-e $f) ? " exists" : " doesn't exist");
    print "\n";

    my $f = $f2;
    utf8::upgrade($f);
    print $f;
    print ((-e $f) ? " exists" : " doesn't exist");
    print "\n";

    my $f = $f2;
    utf8::downgrade($f);
    print $f;
    print ((-e $f) ? " exists" : " doesn't exist");
    print "\n\n";
}
[download]

which produces:

c:\filé.txt exists
c:\filé.txt doesn't exist
c:\filé.txt exists

c:\filé.txt doesn't exist
c:\filé.txt doesn't exist
c:\filé.txt exists
[download]

Is utf8::downgrade always guaranteed to produce a string with the required encoding for any environment though?