my $colname= 'city';
my $value=   'Toledo';

##</code><code>##

select * from mytable where city = 'Toledo';

##</code><code>##

my $sth= $dbh->prepare("select * from mytable where ? = ?");
... $sth->execute( $colname, $value );

##</code><code>##

select * from mytable where 'city' = 'Toledo';