One-liner:

perl -e \
'$_="A3xyz45M98AB7Z9",s/(\d)([a-zA-Z])/$1:$2/gsi,print'
[download]

Using split:

use strict;

my $s = "A3xyz45M98AB7Z9";
my @stuff = split(//,$s);
for (my $j=0;$j<@stuff;$j++) {
  if ($stuff[$j] =~ /\d/ && $stuff[$j+1] && $stuff[$j+1] =~ /[a-zA-Z]/
+) {
    print "$stuff[$j]:";
  } else {
    print "$stuff[$j]";
  }
}
[download]

I'm working on a way to make it as complicated as possible. Just because.

piroufreek