my @ch = (' ', '0'..'9', 'A'..'Z');
my $base = @ch;

my $num = 2_565_726_408;
my $s = '';

while ($num > 0) {
    $s = $ch[$num % $base] . $s;
    $num = int($num / $base);
}

print $s;  # ZZZZZZ
[download]

(for small numbers you could space-pad the string on the left, if you like)

And the reciprocal procedure (string to number) could be

my @val;
$val[ ord($ch[$_]) ] = $_ for 0..$#ch;

my $s = ...
my $num = 0;
my $m = 1;
for (reverse split //, $s) {
    $num += $m * $val[ ord($_) ];
    $m *= $base;
}
print $num;
[download]