since the data files are effectively presorted...

#!/usr/bin/perl -w

use strict;

use Algorithm::Diff qw(traverse_sequences);

die unless @ARGV == 2 and $ARGV[0] ne $ARGV[1];

my @files = @ARGV;

my %data;

push @{$data{$ARGV}}, $_ while <>;

my $a = $data{$files[0]};
my $b = $data{$files[1]};

my (@additions, @deletions);

traverse_sequences $a, $b, {
    DISCARD_A => sub { push @deletions, $a->[$_[0]] },
    DISCARD_B => sub { push @additions, $b->[$_[0]] },
};

if (@deletions)
{
    print "Deletions:", $/;
    print for @deletions;
    print $/;
}

if (@additions)
{
    print "Additions:", $/;
    print for @additions;
    print $/;
}
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