c:\@Work\Perl\monks>perl -wMstrict -le
"use Data::Dump qw(dd);
 ;;
 my @lines = (
   'line 1 this line has no Foo number: keep',
   'line 2 Foo something 1234-12.3 other 123 first valid keep',
   'TOSS 3 Foo toss this 1234-12.3 line 123 DELETE',
   'line 4 Foo not same 1234-12.3 offset 123 line keep',
   'line 5 Foox not tag  1234-12.3 same offset 123 keep',
   'line 6 has no Foo number: keep',
   'line 7 Foo yyyy xxx 1234-12.3 this line keep',
   'line 8 Foo xxxx zzzz 1234-12.4 different n keep',
   'TOSS 9 Foo kill this 1234-12.3 one DELETE',
   'line 10 no Foo number: keep',
   );
 dd \@lines;
 print qq{\n};
 ;;
 my $strange_n = qr{ (?<! \d) \d+ - \d+ (?: [.] \d+)* (?! \d) }xms;
 my $toss_tag  = qr{ \b Foo \b }xms;
 ;;
 my $rx_can_toss = qr{ \A .+ $toss_tag \D* ($strange_n) }xms;
 ;;
 my ($valid_was_seen, $sn, $sn_offset);
 ;;
 my @keep_lines;
 LINE:
 for my $i (0 .. $#lines) {
   my ($sn_seen, $this_sn, $this_sn_offset) =
     map { $_ ? (1, $1, $-[1]) : () }
     $lines[$i] =~ $rx_can_toss
     ;
   if ($sn_seen) {
     if ($valid_was_seen) {
       next LINE if $sn eq $this_sn and $sn_offset eq $this_sn_offset;
       }
     else {
       ($valid_was_seen, $sn, $sn_offset) = (1, $this_sn, $this_sn_offset);
       }
     }
   push @keep_lines, $lines[$i];
   }
 dd \@keep_lines;
"
[
  "line 1 this line has no Foo number: keep",
  "line 2 Foo something 1234-12.3 other 123 first valid keep",
  "TOSS 3 Foo toss this 1234-12.3 line 123 DELETE",
  "line 4 Foo not same 1234-12.3 offset 123 line keep",
  "line 5 Foox not tag  1234-12.3 same offset 123 keep",
  "line 6 has no Foo number: keep",
  "line 7 Foo yyyy xxx 1234-12.3 this line keep",
  "line 8 Foo xxxx zzzz 1234-12.4 different n keep",
  "TOSS 9 Foo kill this 1234-12.3 one DELETE",
  "line 10 no Foo number: keep",
]

[
  "line 1 this line has no Foo number: keep",
  "line 2 Foo something 1234-12.3 other 123 first valid keep",
  "line 4 Foo not same 1234-12.3 offset 123 line keep",
  "line 5 Foox not tag  1234-12.3 same offset 123 keep",
  "line 6 has no Foo number: keep",
  "line 7 Foo yyyy xxx 1234-12.3 this line keep",
  "line 8 Foo xxxx zzzz 1234-12.4 different n keep",
  "line 10 no Foo number: keep",
]