perl 11157166.pl
tempfile.txt is size 29 Mb
0.133742 read lines from disk and do RE (2047 matches).
0.079146 read lines from in-memory file and do RE (2047 matches).

##</code><code>##

tempfile.txt is size 29 Mb
0.057704 read lines from disk and do RE (2047 matches).
0.805546 read lines from in-memory file and do RE (2047 matches).

##</code><code>##

#!/usr/bin/env perl

use warnings;
use strict;
use Time::HiRes qw( time );

my $file = shift @ARGV;
my ($fh, $time);

if (!$file) {
    #  should use Path::Tiny::tempfile
    $file = 'tempfile.txt';
    open my $ofh, '>', $file or die "Cannot open $file for writing, $!";
    
    srand(1234567);
    
    for my $i (0..200000) {
        my $string = 'some random text ' . rand();
        $string = $string x int (rand() * 10);
        if (rand() < 0.01) {
            $string = " Query${string}";
        }
        say {$ofh} $string;
    }
    $ofh->close or die "Cannot close $file, $!";
    printf "%s is size %i Mb\n", $file, (-s $file) / (1028**2);
}


open $fh, "<", $file;
$time = time;
my $match_count1;
while(<$fh>) {
  /^ ?Query/ && $match_count1++;
}
printf "%f read lines from disk and do RE ($match_count1 matches).\n", time - $time;

seek $fh, 0, 0;
my $s = "";
while(<$fh>) {
  $s .= $_;
}

open $fh, "<", \$s;
$time = time;
my $match_count2;
while(<$fh>) {
  /^ ?Query/ && $match_count2++;;
}
printf "%f read lines from in-memory file and do RE ($match_count2 matches).\n", time - $time;