use v5.20;
use Data::Dump "pp";

my @z = glob('{a,b,c}'x6);
my $z = '(?:a()|a()|b()|b()|c()|c()){6}\1\2\3\4\5\6';

for my $j (@z)
{
    $j =~ $z and say pp {$j => \@- };
}
##</code><code>##

...
{ acbcaa => [0, 6, 6, 3, 3, 4, 4] }
{ acbcab => [0, 5, 5, 6, 6, 4, 4] }
{ acbcac => [0, 5, 5, 3, 3, 6, 6] }
{ acbcba => [0, 6, 6, 5, 5, 4, 4] }
{ acbcca => [0, 6, 6, 3, 3, 5, 5] }
{ accaab => [0, 5, 5, 6, 6, 3, 3] }
{ accaba => [0, 6, 6, 5, 5, 3, 3] }
...
{ cccbab => [0, 5, 5, 6, 6, 3, 3] }
{ cccbac => [0, 5, 5, 4, 4, 6, 6] }
{ cccbba => [0, 6, 6, 5, 5, 3, 3] }
{ cccbca => [0, 6, 6, 4, 4, 5, 5] }
{ ccccab => [0, 5, 5, 6, 6, 4, 4] }
{ ccccba => [0, 6, 6, 5, 5, 4, 4] }

##</code><code>##

my @y = glob('{a,b,c}'x6);
my $y = '(?:(?!\1)a()|(?!\2)a()|(?!\3)b()|(?!\4)b()|(?!\5)c()|(?!\6)c()){6}\1\2\3\4\5\6';

for my $j (@y)
{
    $j =~ $y and say $j;
}
##</code><code>##

aabbcc
aabcbc
aabccb
aacbbc
aacbcb
aaccbb
ababcc
abacbc
abaccb
...
ccbaab
ccbaba
ccbbaa
##</code><code>##

# edit reformatted as a multiline regex for clarity
my $y = qr/(?:
               (?!\1) a ()
             | (?!\2) a ()
             | (?!\3) b ()
             | (?!\4) b ()
             | (?!\5) c ()
             | (?!\6) c ()
           ){6}
         /x;