You can shave a bit of processing off by slurping a big chunk of data and then splitting at all the included newlines on your own.

while(read $fh, my $buffer, 128*2**10) {
    $buffer .= <$fh>; # since last line probably crosses buffer border
    for(split /\n/, $buffer) {
        # ...
    }
}
[download]

You can save more processing time by limiting the record split, so that it knows to stop looking even when it hasn't reached the end of the input string. my @field = split /\|/, $_, 10; # each record has 10 fields Potentially much more efficient is a fairly complicated approach - caveat: your input data must not contain any format errors or it'll completely run wild.

# we assume 10 fields per record again
while(read $fh, my $buffer, 128*2**10) {
    $buffer .= <$fh>;

    # we dump all records' fields in a big pile, in which
    # every 9th element contains the last field of one record,
    # plus a newline, plus the first field of the next record
    my @in_field_heap = split /\|/, $buffer;

    while(@in_field_heap) {
        # pull the two glued fields apart
        $in_field_heap[9] =~ /^([^\n]*)\n(.*)/;

        # pull out fields of current record incl the glued one,
        # and reinject the second half of the double field
        my @field = splice @in_field_heap, 0, 10, $2;

        # replace the glued double field by its first half
        @field[9] = $1;

        # ...
    }
}
[download]

A similar approach comes to mind for your output, but my intuition is entirely undecided on whether it'll run faster or slower.

        # ...
        push @out_field_heap, @field, "\n";

    }
    # single join over the whole batch
    my $out_buffer = join "|", @out_field_heap;


    # but that means we surrounded the newlines with pipes,
    # so fix em
    $out_buffer =~ s/\|\n\|/\n/g;

    print OUTPUT $out_buffer;
}
[download]

Obviously, optimization for speed can decrease your code's legibility and maintainability fast. Be wary of whether you really need it.

Disclaimer: I benchmarked none of these. The read $fh, $buffer, $len; $buffer .= <$fh>; is known to be the fastest block slurping approach however.

If that output acceleration idea works, it might well be applicable to the split acceleration as well.

# 10 fields..
while(read $fh, my $buffer, 128*2**10) {
   $buffer .= <$fh>;
   $buffer =~ s/\n/\|/g;
   my @in_field_heap = split /\|/, $buffer;

   while(my @field = splice @in_field_heap, 0, 10) {
       # ...
   }
}
[download]

YMMV. Benchmark thoroughly.

Makeshifts last the longest.

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