#! perl -slw
use strict;
use Math::Random::MT qw[ rand ];
use constant LIMIT => 1 / 40000;

$|++;

my( $aveTrue, $aveFalse ) = (0,0);
while( 1 ) {
    my( $true, $false ) = (0,0);
    for( 1 .. 40000 ) {
        if( rand() < LIMIT ) {
            ++$true;
        }
        else {
            ++$false;
        }
    }
    $aveTrue += $true;
    $aveFalse += $false;
    printf "\rThis time: $true/$false; Accumulate ratio: %f / 40000", $aveTrue / ( $aveFalse / 40000 );
}

__END__
C:\test>1001701.pl
This time: 0/40000; Accumulate ratio: 0.995570 / 40000