next unless/(?=.*a)(?=.*b)(?=.*c)(?=.*d)(?=.*e)(?=.*f)/i;push@_,$_

# ==>
lc=~s/(.)(?=.*\1)//gr=~y/abcdef//<6||push@_,$_

# or, on Perls <5.14:
($b=lc)=~s/(.)(?=.*\1)//g;$b=~y/abcdef//<6||push@_,$_