The simplest way is to redefine what is considered to be a line. That is done by setting $/ to the delimiter text.

my @errors;
{
    local $/ = "=================================\n";
    open my $fh, '<', '/path/to/file.txt' or die $!;
    while (<$fh>) {
        chomp;
        push @errors, $_ if /D : Error Occurred/;
    }
    close $fh or die $!;
}
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You can look up the A and B values with a regex on the raw text in @errors, or else break that into a hash with A..D as keys.