Following up on the earlier replies, if I supplement the OP code like so: 
use Encode;

my $utf8key = "\x{05D0}";
my $usable_key = encode( 'utf8', $utf8key );

dbmopen(my %hash, "/tmp/mydb", 0666) || die "d'oh!";
$hash{$usable_key} = "bar";
dbmclose(%hash);

[download]
I don't get the warning message. I also noticed some differences in the content of the resulting dbm file -- the OP version had null bytes where the 'encoded' version had non-null bytes, suggesting that the warning issued by the OP version reflects an actual failure to store the data.

Having to encode the hash keys like this is certainly a PITA (a minor one, but still). Perhaps the maintainer(s) the various *DBM_File modules can be persuaded to update them so as to handle this properly -- easy enough to do, I'd expect.

In reply to Re: utf-8 keys in a tied hash cause warning by graff
in thread utf-8 keys in a tied hash cause warning by saintmike

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.