TIMTOWTDI :-)

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@_,$_
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Update: s/tr/y/, thanks Eily :-)