use Encode qw( is_utf8 encode_utf8 );

is_utf8($string) ? encode_utf8($string) : $string

##</code><code>##

use Encode qw( encode_utf8 );

$dbh->do("SET NAMES utf8");

my $sth = ...;
$sth->execute(encode_utf8($decoded));

##</code><code>##

use Encode qw( decode );

$dbh->do("SET NAMES utf8");

my $sth = ...;
$sth->execute(decode('cp1252', $encoded));

##</code><code>##

sub _d { my ($s) = @_; utf8::downgrade($_); $s }

$dbh->do("SET NAMES cp1252");

my $sth = ...;
$sth->execute(_d($encoded));