$ cat pm1207805.pl
#!usr/bin/perl
use strict;
use warnings;
use Text::CSV;
use List::Util qw(sum);
use Data::Dumper;

my $csv = Text::CSV->new();
my $FH = \*DATA;
my %accumulator;

# Gather the digits for the cities
while (my $row = $csv->getline($FH)) {
    my $city = $row->[1];
    my $digits = $row->[3];
    push @{$accumulator{$city}}, $digits;
}

# What do we have to work with?
print "Data:\n", Dumper(\%accumulator), "\n\n";

# Dump our results
for my $city (sort keys %accumulator) {
    my $num_rows = @{$accumulator{$city}};
    print "$city: ", sum(@{$accumulator{$city}}) / $num_rows, "\n";
}

__DATA__
CA006139520,"WINDSOR RIVERSIDE, ON CA",2018-01-02,10
CA006139520,"WINDSOR RIVERSIDE, ON CA",2018-01-02,20
CA006139520,"WINDSOR RIVERSIDE, ON CA",2018-01-02,14
CA006138520,"NEW YORK",2018-01-02,11
CA006137520,"PHILADELPHIA, ON CA",2018-01-02,23
CA006137520,"PHILADELPHIA, ON CA",2018-01-02,25
CA006138520,"NEW YORK",2018-01-02,13
CA006138520,"NEW YORK",2018-01-02,19