Couple other additions to assortment of facts:

• if loops are changed to

  while(my $s = <$fh>) {
    $s =~ /^ ?Query/ && $match_count1++;
  }
  [download]

  then deterioration gets significantly worse (compare to parent node):

  v5.32.1
  tempfile.txt is size 29 Mb
  0.121957 read lines from disk and do RE (32571 matches).
  9.511288 read lines from in-memory file and do RE (32571 matches).
  [download]

• if loops are changed to

  while(my $s = <$fh>) {
    index( $s, 'Query' ) != -1 && $match_count1++;;
  }
  [download]

  (the same without the "$s") then using in-memory string is (of course) faster:

  v5.32.1
  tempfile.txt is size 29 Mb
  0.092390 read lines from disk and do RE (32571 matches).
  0.051797 read lines from in-memory file and do RE (32571 matches).
  [download]