# numbered:
my $sth=$dbh->prepare('SELECT ... FROM ... WHERE SUBSTR(column, 1, LENGTH(:1)) = :1');
$sth->bind_param(1, $what);
$sth->execute();

# named:
my $sth=$dbh->prepare('SELECT ... FROM ... WHERE SUBSTR(column, 1, LENGTH(:what)) = :what');
$sth->bind_param(':what', $what);
$sth->execute();

##</code><code>##

my $sth=$dbh->prepare('SELECT ... FROM ... WHERE SUBSTR(column, 1, ?) = ?');
$sth->execute(length($what), $what);