use strict;
use warnings;

use 5.010;

use List::Util qw{ sum };
use Data::Dumper;

open my $windowsFH, q{<}, \ <<EOD or die qq{open: < HEREDOC: $!\n};
10 19 0.287
11 20 0.502
12 21 0.462
13 22 0.407
14 23 0.254
15 24 0.335
16 25 0.474
EOD

my %values;

while ( <$windowsFH> )
{
    my( $min, $max, $value ) = split;
    push @{ $values{ $_ } }, $value for $min .. $max;
}

close $windowsFH or die qq{close: < HEREDOC: $!\n};

print Data::Dumper
   ->Dumpxs( [ \ %values ], [ qw{ *values } ] );

open my $markersFH, q{<}, \ <<EOD or die qq{open: < HEREDOC: $!\n};
12
15
17
EOD

while ( <$markersFH> )
{
    chomp;
    say qq{$_ - }, ( sum @{ $values{ $_ } } ) / @{ $values{ $_ } };
}

close $markersFH or die qq{close: < HEREDOC: $!\n};

##</code><code>##

%values = (
            '25' => [
                      '0.474'
                    ],
            '11' => [
                      '0.287',
                      '0.502'
                    ],
            '21' => [
                      '0.462',
                      '0.407',
                      '0.254',
                      '0.335',
                      '0.474'
                    ],
            '17' => [
                      '0.287',
                      '0.502',
                      '0.462',
                      '0.407',
                      '0.254',
                      '0.335',
                      '0.474'
                    ],
            '12' => [
                      '0.287',
                      '0.502',
                      '0.462'
                    ],
            '20' => [
                      '0.502',
                      '0.462',
                      '0.407',
                      '0.254',
                      '0.335',
                      '0.474'
                    ],
            '15' => [
                      '0.287',
                      '0.502',
                      '0.462',
                      '0.407',
                      '0.254',
                      '0.335'
                    ],
            '14' => [
                      '0.287',
                      '0.502',
                      '0.462',
                      '0.407',
                      '0.254'
                    ],
            '22' => [
                      '0.407',
                      '0.254',
                      '0.335',
                      '0.474'
                    ],
            '18' => [
                      '0.287',
                      '0.502',
                      '0.462',
                      '0.407',
                      '0.254',
                      '0.335',
                      '0.474'
                    ],
            '24' => [
                      '0.335',
                      '0.474'
                    ],
            '23' => [
                      '0.254',
                      '0.335',
                      '0.474'
                    ],
            '19' => [
                      '0.287',
                      '0.502',
                      '0.462',
                      '0.407',
                      '0.254',
                      '0.335',
                      '0.474'
                    ],
            '10' => [
                      '0.287'
                    ],
            '13' => [
                      '0.287',
                      '0.502',
                      '0.462',
                      '0.407'
                    ],
            '16' => [
                      '0.287',
                      '0.502',
                      '0.462',
                      '0.407',
                      '0.254',
                      '0.335',
                      '0.474'
                    ]
          );
12 - 0.417
15 - 0.3745
17 - 0.388714285714286