my $sql = "select ..."; # query string as suggested by Corion
my $sth = $dbh->prepare( $sql );
$sth->execute;

my $row_data = $sth->fetchall_arrayref;  # gets all data from query
my %data_structure;

for my $row ( @$row_data ) {
    my ( $college, $dept, $it, $ip ) = @$row;
    $data_structure{$college}{$dept}{$it} = $ip;
}