Something like this might do you. Essentially everything inside the async will happen concurrently with everything after it until you join the threads back together.

use threads;
use threads::shared;
use Win32::SerialPort;
use Net::FTP;

my $sp = new Wi3n32::SerialPort( ... );
...

my $count = $sp-write( $out );

my $done :shared = 0;
my $t = async{
    my $in;
    until( $done ) {
        my( $count, $byte ) = $sp->read( 1 );
        warn "Read error" and next unless $count = 1;
        $in .= $byte; 
    }
    return $in;
};

my $ftp = new Net::FTP( ... );
$ftp->get( 'theFile' );
$ftp->done;
$done = 1;
my $input = $t->join;
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