Postorder

Score: 50.08 (pass)

#!perl -l
for$}(pop,pop=~/./g){s~$}(.*)|$~
$1
$}~}print//

Score: 50.10 (pass)

#!perl -l
for$~(pop,pop=~/./g){s/$~(.*)|$/
$1
$~/}print//

Score: 52.08 (pass)

#!perl -l
for$~(pop,pop=~/\w/g){s/$~(\w*)|$/ $1 $~/}print//

Score: 55.08 (pass)

#!perl -l
for$~(pop,pop=~/\w/g){s/$~(\w*)|$/ $1 $~/}print/\w/g

Score: 56.08 (pass)

#!perl -l
for$~($_=pop,pop=~/\w/g){s/$~(\w*)/ $1 $~/}print/\w/g

Score: 57.10 (pass)

#!perl -l
s~~@ARGV~;$}.=$1while s~(.)(?=(.).*\2.*\1| )~~;print$}

Score: 58.12 (pass)

#!perl -l
$_="@ARGV";$}.=$1while s~(.)(?=(.).*\2.*\1| )~~;print$}

Score: 59.08 (pass)

#!perl -l
$~=~s/$_(\w*)/ $1 $_/for$~=pop,pop=~/\w/g;print$~=~/\w/g

Score: 61.09 (pass)

#!perl -l
$b=pop;$b=~s/$_(\w*)/ $1 $_/for(pop=~/\w/g);print$b=~/\w/g

Score: 63.09 (pass)

#!perl -l
$b=pop;$_=pop;$b=~s/$_(\w*)/ $1 $_/for(/\w/g);print$b=~/\w/g

Score: 63.10 (pass)

Oops, of course it can be improved...

#!perl -l
$_=join z,@ARGV;$}.=$1while s~(.)(?=(.).*\2.*\1|z)~~;print$}

Score: 67.10 (pass)

#!perl -l
$b=pop;$_=pop;$b=~s/$_(\w*)/ $1 $_/for(/\w/g);$b=~s/ //g;print$b

Score: 74.10 (pass)

Just an interesting solution, I wonder if this can be improved

#!perl -l
$_=join z,@ARGV;$}.=$1while s~(.)(.)(?=.*\2.*\1)~$2~||s~(.)z~z~;print$}