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my $sth = $dbh->prepare(
  'INSERT INTO table('
...
  . chop(join('', map {'?,'} keys %$c))
  . ")"
);

my $placeholderstr = join('', map {'?,'} keys %$c);
chop $placeholderstr;
...
);

$sth->execute(values %$c) or die $dbh->errstr;