I found that part of the spec ambiguous and so made up a convenient definition.

Personally, I'd like something that has a chance of producing a long string, but favouring shorter ones. Hence, a simple 1 + int rand 12 won't do. Rather, I'm tempted to use an exponential function:

$_ = 'cat';
s/(.)/$1 x int(12**rand)/ge;
print;
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caaatttttttt

But that still puts an arbitrary (low) limit on the number of repetitions. The code below doesn't:

#!/usr/bin/perl

use strict;
use warnings;

$_ = 'cat';

s/(.)/my $t = $1; $t .= $1 while 0.95 > rand; $t/ge;

print "$_\n";

__END__
cccccccccccccaaaaat
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Abigail

You would have a larger range of possible numbers with:

$string =~ s=(\w)= $1 x (1 - 1/log(rand)) =eg;
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