use strict;
use warnings;

use 5.010;

say genString( 12, ( q{a} .. q{f} ) );

say genString( 20, ( q{a} .. q{c} ) );

say genString( 20, ( q{a} .. q{b} ) );

sub genString
{
    my( $len, @set ) = @_;
    my $string = q{};
    for ( 1 .. $len )
    {
        if ( $string =~ m{(.)\1$} )
        {
            my $let;
            do { $let = $set[ int rand @set ] } until $let ne $1;
            $string .= $let;
        }
        else
        {
            $string .= $set[ int rand @set ];
        }
    }
    return $string;
}