Threads would be much simpler.

This could be refined in any of a number of ways given a real problem description, but just supplying the filename as an input argument should work:

#! perl -slw
use strict;
use threads ( stack_size => 4096 );
use threads::shared;
use Thread::Queue;

sub process {
    my $tid = threads->tid;
    my( $hashref, $Q ) = @_;
    while( my $key = $Q->dequeue ) {
        print "$tid : Processing $key";
        sleep 10;
        lock $hashref;
        ++$hashref->{ $key };
    }
}

our $T //= 16;
my %hash :shared;
my $Q = new Thread::Queue;

my @t = map threads->create( \&process, \%hash, $Q ), 1 .. $T;

while( my $key = <> ) {
    chomp $key;
    if( exists $hash{ $key } ) {
        $Q->enqueue( $key );
    }
    else {
        $hash{ $key } = 0;
    }
}

$Q->enqueue( (undef) x $T );
$_->join for @t;
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