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$a.$b.$c.$d

concat_assign(  
   concat_assign( 
...
   ),
   $d
)

concat_assign(  
   concat_assign( 
...
   ),
   $d
)

$ perl -MO=Concise,-exec -e'$a.$b.$c.$d'
1  <0> enter 
...
9  <2> concat[t7] vKS/2
a  <@> leave[1 ref] vKP/REFC
-e syntax OK

sub concat_list :lvalue {
   my $tot_len;
...
   $buf .= $_ for @_;
   $buf
}