Thank you, huck++.

Just completed testing on WIndows, Linux, and Mac OS. We have a UDP solution that works on multiple platforms. Fortunately, Perl provides $^X, giving us the path to the perl executable.

use strict;
use warnings;

use Fcntl qw( F_GETFL F_SETFL O_NONBLOCK );
use IO::Socket::INET qw( SOL_SOCKET SO_REUSEADDR );

## POSIX.pm is a big module. This covers most platforms.
use constant { _WNOHANG => $^O eq 'solaris' ? 64 : 1 };

my $port = 54321;

my $socket = IO::Socket::INET->new(
    Proto     => 'udp',
    LocalPort => $port,
    LocalAddr => '127.0.0.1'
);

die "Unable to bind to 127.0.0.1:$port: $!\n" unless $socket;

setsockopt( $socket, SOL_SOCKET, SO_REUSEADDR, 1 );
stop_blocking( $socket );

$SIG{CHLD} = 'IGNORE'; my $pid = fork;
die "Could not fork: $!\n" unless ( defined $pid );

exec( $^X, 'monitored.pl', $port ) if ( $pid == 0 );

$| = 1;

while ( waitpid($pid, _WNOHANG) == 0 ) {
    my $addr = recv( $socket, my $time, 1024, 0 ) or select '','','',0
+.015;

    printf "\rProgress: %s", $time;
}

print "\nAll done.\n";

sub stop_blocking {
    my $socket = shift;

    if ($^O eq 'MSWin32') {
        my $flag = 1; ioctl($socket, 0x8004667e, \$flag);
    }
    else {
        my $flags;
        $flags = fcntl($socket, F_GETFL, 0)
            or die "Could not getfl: $!\n";
        $flags = fcntl($socket, F_SETFL, $flags | O_NONBLOCK)
            or die "Could not setfl: $!\n";
    }
}
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Not changed from before, the monitored.pl script using IO::Socket::INET.

use strict;
use warnings;

use IO::Socket::INET qw( pack_sockaddr_in inet_aton );

my $port    = shift || 54321;
my $socket  = IO::Socket::INET->new( Proto => 'udp' );
my $runtime = rand 10;

$| = 1;

for my $num ( 0 .. $runtime * 100 ) {
    send ( $socket, sprintf("%.f%% ", $num / $runtime),
           0, pack_sockaddr_in($port, inet_aton('127.0.0.1'))
    ) or  die "send: $^E";

    select '', '', '', 0.015;
}
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