Update: OP mentions searching for a keyword. Changed the first if statement to do index not regex.

Parallelism is beautiful. IO reads against the input file are sequential in MCE no matter the number of workers. Workers communicate the next offset position for the next worker waiting to read. The chunk_id value makes it possible to have ordered output.

Well, here is a parallel implementation for the OP script. Chomping isn't necessary after all. But, please add if necessary. But why chomp and later append the line feed when time to output. Perhaps the OP script is in the form of art and chomp was met to be there. If that's the case, I apologize for taking chomp out. :-)

The initial pattern matching against the slurped chunk is likely beneficial. But, please feel free to comment out the outer-most if statement and closing brace and give it a try. Scripting is fun.

Kind regards, Mario

use strict;
use warnings;

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

# ensure two arguments are provided to the script
my ($fileName,$keyword) = @ARGV;
die "usage: $0 file keyword\n" if @ARGV != 2;

# grab header line
open INFILE, "<$fileName" || die "cannot open file to read: $!";
my $header = <INFILE>;
close INFILE;

# utilize many core engine to filter file
# out_iter_array returns a closure for gathering orderly

my @rawData;

MCE::Loop::init {
    max_workers => 'auto', # note: 'auto' is never higher than 8
    gather => MCE::Candy::out_iter_array(\@rawData),
    use_slurpio => 1,
};

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

    # quickly determine if the keyword is found; this is fast
    # think of this as short-circuiting unnecessary work

    my ($count, $foundData) = (0, '');

    if ( 1 + index($$slurped_ref, $keyword) ) {
        open my $MEM_FH, '<', $slurped_ref;
        binmode $MEM_FH, ':raw';

        # skip header line for the first chunk only
        if ($chunk_id == 1) {
            while (<$MEM_FH>) { if (/$keyword/) {
                next if $. == 1;   # skip header line
                $foundData .= $_;  # append line
                $count++;          # increment count
            }}
        }
        # otherwise, the line number check is not necessary
        else {
            while (<$MEM_FH>) { if (/$keyword/) {
                $foundData .= $_;  # append line
                $count++;          # increment count
            }}
        }

        close $MEM_FH;
    }

    # gathers two elements; count and rawData in anonymous array

    # gather must be called irregardless if found or not found
    # the manager process needs to know if this chunk_id has completed
    # when gathering results orderly

    MCE->gather($chunk_id, [ $count, $foundData ]);

} $fileName;

MCE::Loop::finish;    # shutdown MCE workers

# each element in rawData is an array ref [ $count, $foundData ]
# output count

my $filterCount=0;
$filterCount += $_->[0] for @rawData;  # $count

print "Completed filtering $keyword\n";
print "Found $filterCount elements\n";

# output found data

my $outFileName = substr($fileName,0,length($fileName)-4)."_filter.txt
+";
print "Filtering to output file: $outFileName\n";

open  OUTFILE, ">$outFileName" || die "cannot open file to write: $!";
print OUTFILE $header;
print OUTFILE $_->[1] for @rawData;    # $foundData
close OUTFILE;
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