m/
    (?=^[^a]*a[^a]*$) ## One a and no other a's
    (?=^[^b]*b[^b]*$) ## One b and no other b's
    (?=^[^c]*c[^c]*$) ## One c and no other c's
    ^.{3}$            ## Exactly three chars
/x and print "$_: matched" for 'aaa'..'ccc';;

abc: matched
acb: matched
bac: matched
bca: matched
cab: matched
cba: matched