my @ff_fields       = qw( callno lname fname );
my @incident_fields = qw( callno type date hr );

my %incidents_by_callno;
while (@incidents) {
    my %incident;
    @incident{@incident_fields} =
        splice(@incidents, 0, 0+@incident_fields); 
    push @{ $incidents_by_callno{ $incident{callno} } }, \%incident;
}

while (@ffs) {
    my %ff;
    @ff{@ff_fields} = splice(@ffs, 0, 0+@ff_fields);
    print("@ff{@ff_fields}\n");
    for my $incident (@{ $incidents_by_callno{ $ff{callno} } }) {
        print("    @$incident{@incident_fields}\n");
    }
}