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use List::Util qw( sum);

...
    $_ = @$_ ? sum( @$_)/@$_ : '';
}
print "$_ - $prod{ $_}\n" for @emp_lst;

while ( <DATA> ) {
    my ( $emp) = m{<emp>(\d+)</emp>} or
...
    push @emp_lst, $emp unless $prod{ $emp};
    push @{ $prod{ $emp} }, m{<prod>([.\d]+)</prod>};
}