$dbh->do('INSERT INTO users (name, day, phone) VALUES($name, $day, $number)');

##</code><code>##

my $bill = 'Text plus $ted'; # no interpolation.  
my $bill = "Text plus $ted"; # Interpolation occurs.

##</code><code>##

my $sth = $dbh->prepare('INSERT INTO users (name, day, phone) VALUES(?,?,?)' );
$sth->execute( $name, $day, $number );