I do appreciate the help on this issue, it has been wonderful. I have been intrigued by Perl and what it can do. It is definitely quite different from anything that I have used before. I did have to dive into the solutions that were provided a little, because it was not working right out of the gate for me, and it came down to leading spaces in a line of the input in front of "Interpolated Hydrograph". For some reason I left that off on the original post.

I modified that, added a header row, and removed the \r on the MCE->gather so there are no blank lines. The time difference is night and day:

Batch (~275k lines) = ~3 hours

Perl <1 second

The final code that works like a charm that I am using:

use strict;
use warnings;

use MCE::Loop;
use MCE::Candy;

my $input_file  = shift || 'input.txt';
my $output_file = shift || 'output.txt';

my $match_string = "                         INTERPOLATED HYDROGRAPH A
+T   ";

open my $ofh, ">", $output_file
   or die "cannot open '$output_file' for writing: $!\n";
print $ofh "HEC1_ID,Q100,V100\n";
   
MCE::Loop::init {
   use_slurpio => 1, chunk_size => 1, max_workers => 4,
   gather => MCE::Candy::out_iter_fh($ofh),
   RS => "\n${match_string}",
};

## Below, each worker receives one record at a time
## Output order is preserved via MCE::Candy::out_iter_fh

## line  1 CAC40 #  INTERPOLATED HYDROGRAPH AT CAC40
## line  2       #  blank line here
## line  3       #  PEAK FLOW TIME MAXIMUM AVERAGE FLOW
## line  4       #  6-HR 24-HR 72-HR 166.58-HR
## line  5       #  + (CFS) (HR)
## line  6       #  (CFS)
## line  7 1223. #  + 1223. 12.67 890. 588. 245. 106.
## line  8       #  (INCHES) .154 .408 .509 .509
## line  9 1456. #  (AC-FT) 441. 1166. 1456. 1456.
## line 10       #  CUMULATIVE AREA = 53.67 SQ MI

mce_loop_f {
   my ( $mce, $chunk_ref, $chunk_id ) = @_;

   ## Skip initial record containing header lines including *** ***

   if ( $chunk_id == 1 && $$chunk_ref !~ /^${match_string}/ ) {
      ## Gathering here is necessary when preserving output order,
      ## to let the manager process know chunk_id 1 has completed.
      MCE->gather( $chunk_id, "" );
      MCE->next;
   }

   ## Each record begins with INTERPOLATED HYDROGRAPH.

   my ( $k1, $k2, $k3 ) = ( "", "", "" );

   open my $ifh, "<", $chunk_ref;

   while ( <$ifh> ) {
      $k1 = $1 and next if $. == 1 && /(\S+)\s*$/;
      $k2 = $1 and next if $. == 7 && /^\S+\s+(\S+)/;
      $k3 = $1 and last if $. == 9 && /(\S+)\s*$/;
   }
   close $ifh;

   ## Gather values.

   MCE->gather( $chunk_id, "$k1,$k2,$k3\n" );

} $input_file;
[download]

Thanks again. I hope to be learning more of this in the future.