Just to chime in with my own version,

1-10, 500x:  2 secs ( 1.13 usr  0.00 sys =  1.13 cpu) (printing)
1-10, 500x:  1 secs ( 0.48 usr  0.00 sys =  0.48 cpu) (not printing)
1-35m  10x: 26 secs (26.53 usr  0.00 sys = 26.53 cpu) (printing)
1-35,  10x: 15 secs (15.05 usr  0.00 sys = 15.05 cpu) (not printing)
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My approach is similar to several of the ones already posted, but differs enough that I figured I might as well post it:

$x = 1;
for(1..10){
        $x =~ s/((.)\2*)/length($1).$2/ge;
        #print "@{[split//,$x]}\n";
    }
[download]

Uncomment that print line to print everything out. And, of course, get rid of the array-de-ref function call if you don't care about spaces between the numbers.

Incidentally, I used the array ref so I could take advantage of $" being a space and not have to worry about mapping the numbers so they'd have spaces between them.