Note that that error is not directly because the hash key is invalid. This would produce the same error:

my $c = undef;
print "$c\n";
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Anon's sample only emits a warning because it's using the bad value for something that doesn't accept undef's... but if you were doing something like the following, you would get silent failure/undesired results:

my $ct = $dbh->selectrow_array("SELECT ... WHERE x = ?",{},$c);
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