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};