#! perl -slw
use strict;

my @ports = ( 50000..51545,51549,51555..51999, 65535 );

my $vec = '';
vec( $vec, $_, 1 ) = 1 for 0 .. 50_000, @ports;

sub nextFreePort {
    no warnings 'portable';
    my $open_port;
    my $t = int( length( $vec ) / 8 );
    my $n = 782;
    ++$n while $n <= $t and vec( $vec, $n, 64 ) == 18446744073709551615;
    $n *= 64;
    vec( $vec, $_, 1 ) or $open_port = $_ and last for $n .. $n + 64;
    vec( $vec, $open_port, 1 ) = 1;
    return $open_port;
}

my $next_port = nextFreePort();
print $next_port;

__END__
c:\test>899314.pl
51546