If knowing the position of the port number in the string is necessary in order to be able to work your way back to and extract the index number, there's a more direct way:

>perl -wMstrict -le
"my $dd = qr{ \d{1,2} }xms;
 ;;
 my %indices;
 for my $s (
   'ME170-5/2/8-ME172-2/2/6-ME4028-FOO172-222/2/666-BAR',
   'ME172-2/1/2-ME196-1/1/33-ME4002-11/2/3-ME1-1/22/3-ME22',
   '1-1/2/3', 'ME-9/9/9-ME',
   ) {
   push @{ $indices{$1} }, $2
     while $s =~ m{ (\d+) - ($dd (?: / $dd){2}) }xmsg;
   }
 ;;
 for my $i (sort { $a <=> $b } keys %indices) {
   printf qq{index '$i': port(s) '%s' \n},
          join q{' '}, @{ $indices{$i} };
   }
"
index '1': port(s) '1/22/3' '1/2/3'
index '170': port(s) '5/2/8'
index '172': port(s) '2/2/6' '2/1/2'
index '196': port(s) '1/1/33'
index '4002': port(s) '11/2/3'
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