How about 
# variation with grep
@a=qw(abc def ghi jkl mno pqr stu vwxyz); 
@g = grep /i/../u/, @a; 
for (1..$#g-1) { 
  print "$g[$_]\n"; 
}

# with map
@a=qw(abc def ghi jkl mno pqr stu vwxyz); 
$flag=-1; 
sub go {
  print "$_\n" if ($flag>0); 
  if ($_ =~ /i|u/) {
    $flag = $flag * -1
  }
} 
@g = map{ &go($_) => $_ } @a;

[download]
Just checked, it does not work if you say qw(abc start some things to print stop vwxyz) and look for start|stop but maybe that's not how you're using it. Does seem to work (checked the grep version) for qw(abc def start ghi jkl xxx mno pqr stu stop vwxyz).
Updated: You guys are so right. Kudos and votes to you.

In reply to Re: flip-flop operator and sequence number by mattr
in thread flip-flop operator and sequence number by iakobski

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