gistfile1.pl

use strict;
use warnings;
use IO::Socket::INET;

sub can_bind {
    my $port = shift;
    !! IO::Socket::INET->new(
        Listen    => 5,
        LocalAddr => '127.0.0.1',
        LocalPort => $port,
        Proto     => 'tcp',
        ReuseAddr => 1,
    );
}

sub empty_port {
    for (my $port = 49152; ; ++$port) {
        if (can_bind($port)) {
            warn "found empty_port: $port, the socket should have been closed now that we are out of can_bind";
            return $port;
        }
    }
}

my $s = IO::Socket::INET->new(
    Listen    => 5,
    LocalAddr => '127.0.0.1',
    LocalPort => empty_port(),
    Proto     => 'tcp',
    ReuseAddr => 1,
) or die "could not bind to the returned port:$!";

print "ok\n";