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[QUOTE="Dr.Ruud, post: 4135238"]
The input is a string on STDIN,
and the output is a string on STDOUT?

Use a hash:

perl -MData::Dumper -wle '$Data::Dumper::Sortkeys = 1;
   my $t = "((0 a b) (1 c d) (2 e f) (3 g h) (1 i j)"
         . " (2 k l) (4 m n) (2 o p) (4 q r) (5 s t))";

push @{ $h{ $1 } }, $2 while $t =~ /(\w+)([^)]*)/g;  # gist

print Dumper \%h;
'

or an array:

perl -wle '
   my $t = "((0 a b) (1 c d) (2 e f) (3 g h) (1 i j)"
         . " (2 k l) (4 m n) (2 o p) (4 q r) (5 s t))";

push @{$a[$1]},$2 while $t =~ /(\w+)\s+([^)]*)/g; # gist.1
   print "((".join(") (",map join(" ",@$_),@a )."))";  # gist.2
'

Or if the list is not just a string, but a real data structure in the 
script:

perl -wle'
   my $t = [ [qw/0 a b/], [qw/1 c d/], [qw/2 e f/], [qw/3 g h/],
             [qw/1 i j/], [qw/2 k l/], [qw/4 m n/], [qw/2 o p/],
             [qw/4 q r/], [qw/5 s t/] ];

push @{ $a[ $_->[0] ] }, [ @$_[ 1, 2 ] ] for @$t;  # AoAoA

printf "((%s))\n", join ") (",
                        map join( " ",
                          map join( " ", @$_ ), @$_
                        ), @a;
'

Etc.
[/QUOTE]

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