Below, using the MCE::Flow model for ease of passing MCE options and code block. This will increase performance for $input1 by 2.5x or more.

use MCE::Flow;

my $input1 = shift @ARGV;
my @lengths;

mce_flow_f {
   chunk_size => '2m', max_workers => 'auto',
   use_slurpio => 1, gather => \@lengths
},
sub {
   my ($mce, $slurp_ref, $chunk_id) = @_;
   my (@chunk_lengths);

   open my $IN1, '<', $slurp_ref;
   while (<$IN1>) {
      chomp;
      push @chunk_lengths, length $_ unless />/;
   }
   close $IN1;

   MCE->gather(@chunk_lengths);

}, $input1;

MCE::Flow::finish;

print scalar @lengths, "\n";
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