If you only want to find the first free port, you could just connect and see if that fails or use a combination with a timeout (see alarm) for the more complicated cases, e.g. where the server does connect but not respond.

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


sub get_sock_to_free_port {
  my ( $rpt_host, @rpt_ports )  = @_;

  foreach my $rpt_port ( @rpt_ports ) {
    my $sock = new IO::Socket::INET ( PeerAddr => $rpt_host,
                                      PeerPort => $rpt_port,
                                      Proto    => 'tcp'
                                    );
    return $sock if $sock;
    warn "failed check: $rpt_port - $!";
  }
  die "No active port in list: @rpt_ports";
}


my @rpt_ports = qw(12345 12346 22345 22346 22347 22348 22349 22350 
                   22351 22352 22353 22354 22355 22356 22357 22358);

my $sock      = get_sock_to_free_port( '127.0.0.1', @rpt_ports );
...
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Update: (in response to AM feedback below): Premature end of script headers is a problem that can occur after a valid $sock has been found (protocol problem (\r\n?) / server problem / config problem). So it is not directly related to the solution given above. Perhaps checking the HTTPD server logs or the server-script helps?