The Read only error accurred on line 868 which is : my %uniq; could someone explain to me what the last 3 line is doing line by line?

Yes but you have more problems as I stated above;

my %uniq;
# create a hash called %uniq

undef @uniq{@dist_list};
# takes the array @dist_list as keys to create in %uniq
# each with the value undef.
# Since you can only have 1 unique key in a hash this
# has the effect of filtering out repeats
#ex.
#
# %uniq = {
#          '1 2 3' => undef,
#          '4 5 6' => undef,
#          '5 6 7' => undef
#        };


@dist_list = keys %uniq;
# this pulls the keys from %uniq and assings them to @dist_list
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So this code is not a problem - you need to fix the other problems first.