You can't match across multiple lines here, because the while is reading the file one line at a time, so your regexp never sees more than one line.

You either need to slurp the whole file in at once, or try a different approach. Personally I prefer to use the .. operator for this type of processing...

while(<>) {
  if(/Start/ .. /End/) { print }
}
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Or, slurp in the whole file...

{
  local $/;
  my $input = <>;
  if(/ Start(.*)? End/sg) { print $1 }
}
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