I notice you already got answers on how to tell when it flips or filter out the first/last without retesting, but I wanted to make a note on this specific point of repreating the RE.

my $start= qr/start/;  # note the regex's once
my $stop= qr/stop/;
foreach (grep /$start/ .. /$stop/, @data) {
   next if /$start/;
   last if /$stop/;
   # ....
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