And, not just by the way, read How do I post a question effectively? and Markup in the Monastery for some additional guidance on posting questions.

This seems to do what you want (although your specifications are a bit vague, at least, so I followed your description as close to the letter as I could). Note that I wrote this up while tired (worked over 9 hours during a hectic night at the restaurant) and inebriated (had a couple of drinks afterwards) so it can't be that hard :) Then again, I admit this may not be the most elegant way of going about it, but it does what you want and it should get you on the right track. Even if it's an XY problem.

use strict;
use warnings;

my $file1 = << "EOF";   # I'm too lazy to create the actual files...
>A

Amber, Amy

>B

Barbie, Bambie
EOF

my $file2 = << "EOF";   # So I'll just define them as heredocs hre...
>A

Austin, Aurora

>C

Cathy, Candy
EOF

my $file3 = << "EOF";   # and read from them in the next part of the c
+ode.

>B

Bob, Barbara

>C

Cane, Carter
EOF


my @files = (\$file1, \$file2, \$file3);    # References to the scalar
+s above.
my %names = ();                             # Oh, you know... a hash.

for my $f (@files) {                        # Iterate through the refe
+rences
    open my $fh, "<", $f;                   # "Open" them for reading.
    my $key = "";                           # We don't have a key yet.
    my %tmp_names = ();                     # Just a temporary hash...
    $tmp_names{$_} = ["-","-"] for qw(A B C); # ... for the blanks.
    while (<$fh>) {                         # Read lines.
        chomp;                              # I don't care about newli
+nes.
        if (m/^>([ABC])$/) {                # Is it ">" + A, B, or C?
            $key = $1;                      # Then it's a key for the 
+hash.
            next;
        } elsif ($key                       # Do we have a key yet?
                      && $_) {              # And it isn't a blank lin
+e?
            my @names = split(/, /, $_);    # Then it's a list of name
+s!
            $tmp_names{$key} = \@names;     # Store them in the tempor
+ary hash
        }
    }
    close $fh;

    for (qw(A B C)) {
        push @{$names{$_}}, @{$tmp_names{$_}}; # Move tmp hash to real
+ hash.
    }
}

for (qw(A B C)) {
    print ">$_\n\n";
    print join(", ", @{$names{$_}}), "\n\n";
}
[download]

I don't know if this is a better or worse solution, but I'm showing you mine (filenames are only an example, chage them if you have to):

#!/usr/bin/perl

use strict;
use warnings;

# Constants
my $BLANK_LINE = qr/^\s+$/;
my $KEY = qr/>([A-Z]{1})/;

# Variables
my %content;

# Take the data
for (<*.db>) {
    open(FILEDB, $_) or die "Cannot open file $_: $!\n";
 
    my $key;
    for (<FILEDB>) {
        if (/$BLANK_LINE/) {
            next;
        } elsif (/$KEY/) {
            $key = $1;
            if (not $content{$key}) {
                $content{$key} = ();
            }
        } else {
            chomp;
            push (@{$content{$key}}, $_);
        }
    }
 
    close(FILEDB);
}

# Write the new file
open(FILEDB, ">result.db") or die "Cannot open file result.db: $!\n";

for my $key (sort keys %content) {
    print FILEDB ">$key\n\n" . join(", ", @{$content{$key}}) . "\n\n";
}

close(FILEDB);
[download]

Two comments to this, though.

The {1} quantifier in my $KEY = qr/>([A-Z]{1})/; doesn't serve any purpose.

But more importantly, your solution doesn't offer the dashes for those cases where no records were found for a given letter - something which is part of the original specification.

doesn't serve any purpose.

Think of it as a comment, like

my $Count = scalar @sausages;
[download]

scalar isn't required to force scalar context, since the context is already scalar , as opposed to list context where it is required

my($Count) = scalar @sausages;
[download]

, but it helps the programmer remember

The {1} quantifier in my $KEY = qr/>(A-Z{1})/; doesn't serve any purpose.

Mmmm... Yes, I know. I promise I had a good reason to do this, but I cannot remember it right now. Maybe I misunderstood for a moment the greedy effect, I was tired :-).

...your solution doesn't offer the dashes for those cases where no records were found for a given letter

I thought the dashes were a way to specify there could be more data not specified in the example, like ellipsis. Moreover, I saw a similar effect with spaces and I "fixed" it; it's not hard to add them, if they're needed.