More TMTOWTDI... Assuming your BH is predictable and has a static component...

FileStart
Batch: 123
234
1235613246
1434312
12521
124215
Batch: 133
614
1641
32463
142351
123
Batch: 358
214
125
612
FileEnd

Then you could set you Inpute Record Seperator to $/="Batch" and get each batch as a chunk, then process each chunk individually.

open IN, "somefile.txt";
$fileheadinfo = <IN>;
$/ = "Batch";
while ($batch = <IN>) {
    next if $batch = "Batch"; #first line
    @lines = split /\n/,$batch;
    $batchinfo = "Batch" . shift @lines; #get batch info
    pop @lines; #get rid of bar Batch at end
    foreach $line (@lines) {
       process($line)
    }
}
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