Below is some sample code using String::Random to generate random strings of a certain length. It then removes invalid duplicates and appends new random strings until the password passes without duplicates. This assumes you are only working with ascii.

use String::Random;

my $length = 15;
my $Random = new String::Random;
my $password = '';
while (length($password) < $length)
{
   my $missing_length = $length - length($password);
   my $added_string = $Random->randpattern('.' x $missing_length);
   $password .= $added_string;
   $password =~ s/[A-Za-z]{3,}|\d{3,}|[^A-Za-z\d]{3,}//;
}
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