Quote

$mech->field( $name, $value, $number )
$mech->field( $name, \@values, $number )

Given the name of a field, set its value to the value specified. This applies to the current form (as set by the "form_name()" or "form_number()" method or defaulting to the first form on the page).

The optional $number parameter is used to distinguish between two fields with the same name. The fields are numbered from 1.