For clarity more than for speed, 

my $offset = splice @cPosns, rand @cPosns, 1;

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should be 
my $offset = $cPosns[rand @cPosns];

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and 
while (1) {
   ...
   last;
}

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should simply be 
...

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For speed, don't copy the zazb argument into a local variable. Access $_[3] directly.

A possible tweak: 

push @cPosns, pos $zazb while $zazb =~ m{(?=c)}g;

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might be faster as 

push @cPosns, $-[0] while $zazb =~ m{c}g;

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or maybe even 

my $pos = -1;
push @cPosns, $pos while ($pos = index($zazb, 'c', $pos+1)) >= 0;

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Benchmark and find out.

Which I just can't seem to get my head around to get it to work without throwing 'undefined value' warnings all over the place.

This will occur when 'c' doesn't occur in $zazb. Based on your first snippet, you want 

sub popnum3 {
    my ( $x, $y, $z ) = @_;  # $_[3] is $zazb
    if ( $y == 0 ) {
        $aob[$x][0] = $initial * ( 1 + $z );
    }
    else {
        my @cPosns;
        push @cPosns, $-[0] while $_[3] =~ m{c}g;
        if (@cPosns) {
            my $offset = $cPosns[rand @cPosns];
            $aob[$x][$y] = $aob[$offset][ $y - 1 ] * ( 1 + $z );
        }
    }
    return $aob[$x][$y];
}

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In reply to Re: Yet more While issues by ikegami
in thread Yet more While issues by Dandello

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