use strict;
use warnings;

my $file1 = <<END_FILE1;
0: 0,11 5,6 11,2
1: 1,3
3: 0,1 2,2 3,2
5: 3,5 6,1 8,16 9,1
END_FILE1

my $file2 = <<END_FILE2;
0: 0,10 4,19
2: 1,3 2,5 6,4
5: 6,10 9,3
END_FILE2

my %sites;

for my $source (\$file1, \$file2) {
    open my $inFile, '<', $source or die "Failed to open $source: $!\n";

    while (my $line = <$inFile>) {
        chomp $line;
        
        next if ! ($line =~ s/^([^:]+):\s*//);

        my $word = $1;

        for my $field (split ' ', $line) {
            my ($site, $count) = split /,/, $field;

            $sites{$word}{$site} += $count;
        }
    }
}


for my $word (sort {$a <=> $b} keys %sites) {
    print "$word: ";
    
    my $wordSites = $sites{$word};
    
    for my $site (sort {$a <=> $b} keys %$wordSites) {
        print "$site,$wordSites->{$site} ";
    }
    
    print "\n";
}
##</code><code>##

0: 0,21 4,19 5,6 11,2 
1: 1,3 
2: 1,3 2,5 6,4 
3: 0,1 2,2 3,2 
5: 3,5 6,11 8,16 9,4