It would really help if there were some = equal signs separating data3 and header4. Seems like File::Stream doesn't handle lookaheads that well. Nevertheless, here's an example that may help:
use File::Stream;

my $lookahead_regex = qr/\w+[=:]/;

my ($handler, $stream) = File::Stream->new(
    *DATA,
    separator => qr/\n=*\n$lookahead_regex/,
);

my $lookahead = "";
while (my $block = <$stream>)
{
    $block =~ s/($lookahead_regex)$//;
    $block = $lookahead . $block;
    $lookahead = $1;

    print $block;
    print "-" x 60, "\n";
}

__END__
header1=val1
header1b=val1b

data1

==================
header2: val2
header2b: val2b

data2

===============
header3: val3
header3b: val3b

data3

header4= val4
header4b= val4b

data4

[download]

Output:
header1=val1
header1b=val1b

data1

==================
------------------------------------------------------------
header2: val2
header2b: val2b

data2

===============
------------------------------------------------------------
header3: val3
header3b: val3b

data3

------------------------------------------------------------
header4= val4
header4b= val4b

data4
------------------------------------------------------------

[download]

In reply to Re^3: parse file per customized separator / block / metadata by repellent
in thread parse file per customized separator / block / metadata by raiten

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