Just add the DATA :)

#! perl -slw
use strict;
use threads stack_size => 4096;
use threads::Q;

sub thread {
    my $Q = shift;

    while( my $item = $Q->dq ) {
        system qq[ doit.exe $item ];
    }
}

our $T //= 20;

my @ordered_arguments = <DATA>;
chomp @ordered_arguments;

my $Q = threads::Q->new( $T * 2 );

my @threads = map async( \&thread, $Q ), 1 .. $T;

$Q->nq( $_ ) for @ordered_arguments;
$Q->nq( (undef) x $T );

$_->join for @threads;

__DATA__
...
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