I'm going to assume you want something to get picked each time, so you want their probabilities to reflect their relative frequency values. In that case, in pseudo-code:

loop through the lines
  parse out the key and the frequency
  save the key and frequency as a two-element array inside an array
  keep a running total of the frequencies
set an accumulator to 0
loop through the array
  replace each frequency value with a value representing the frequency
         plus the accumulator, divided by the total of the frequencies
  add the original frequency to the accumulator
get a random positive number less than 1
loop through the array one more time
  if the random number is less than the relative frequency value 
    we've found our key, print it and exit the loop
[download]

If that makes sense to you, try that and ask if you need help, showing the code you have so far.

Thanks very much, Aaron B. I got that to work - it's clever. Best wishes, Eric

Your question is very unclear. Are you saying that you want to pick 'A': 1 in a million picks; 'B': 1 in every 20 million picks; and 'C': 1 in every 100,000 picks?

If so, what do you want to pick the other 89,286 times out of every 100,000 picks?

I think you need to clarify your question.

Hi BrowserUK, Sorry about the lack of clarity. I meant these to be relative frequencies, with something getting picked every time.

Then something like this should do the trick:

#! perl -slw
use strict;
use Data::Dump qw[ pp ];

sub genPicker {
    my $fh = shift;
    my( @vals, @odds ); ( $vals[ @vals ], $odds[ @odds ] ) = split( ' 
+' ) for <$fh>;

    ## Sort if not sorted
    my @order = sort{ $odds[ $a ] <=> $odds[ $b ] } 0 .. $#odds;
    @odds = @odds[ @order ];
    @vals = @vals[ @order ];

    ## Calculate and accumulate break points
    my $t = 0; $t += $_ for @odds;
    $_ /= $t for @odds;
    $odds[ $_ + 1 ] += $odds[ $_ ] for 0 .. $#odds - 1;

    ## Generate a subroutine to do the picking
    return sub {
        my $r = rand();
        $r < $odds[ $_ ] and return $vals[ $_ ] for 0 .. $#odds;
    };
}

my $pick = genPicker( *DATA );

## run a quick test
my %tally; ++$tally{ $pick->() } for 1 .. 10e6;

pp \%tally;

__DATA__
A 1e-7
B 20e-7
C 10e-5
[download]

Produces:

C:\test>1127420
{ A => 9949, B => 195307, C => 9794744 }

C:\test>1127420
{ A => 10077, B => 196613, C => 9793310 }
[download]

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