$" = '';
$\ = "\n";

chomp( my @edges = map /--/
                       ? join( '', sort /\w/g )
                       : (),
                   <DATA> );
my @nodes = sort { $a cmp $b }
            uniq( map /\w/g,
                  @edges );

for $node ( @nodes ) {
    my $edge_rx = qr/$node/;
    my @other_nodes = map /$edge_rx/
                          ? grep( $_ ne $node, # ne vs gt
                                  split( //, $_ ) )
                          : (),
                      @edges;
    
    use re 'eval';
    my $third_edge
      = qr/([@other_nodes])
           .*
           ([@other_nodes])
           (??{push @TRI, join(" - ",sort($node,$1,$2))})/x;

    # removed the gt. Now nodes are unordered in the triangle. All triangles are visited three times, not just once.
    grep ! /$node/
         && /$third_edge/,
        @edges;
}
print for uniq( sort @TRI );

sub uniq {
    my %seen;
    grep !$seen{$_}++, @_
}
__DATA__
graph triangles {
  A -- B
  A -- C -- F -- I
  A -- D -- G -- J
  A -- E
  B -- C -- D -- E
  B -- F -- H -- J
  B -- I
  E -- G -- H -- I
  E -- J
  I -- J
}