It is possible to modify the code to get an Out of Memory error on MSYS2 5.36 and Strawberry Perl 5.20 and 5.38. This does not occur with a perlbrewed 5.36 on Ubuntu via WSL, nor Strawberry Perl 5.18.

Collating an array of lines from the in-memory file handle is sufficient. Commenting out the regex in the in-memory file handle loop makes the OOM go away, as does modifying the string before adding it to the array.

(I also modified the code to use a separate variable for the in-memory file handle. It has no effect but is arguably cleaner.)

#!/usr/bin/env perl

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


my $file = shift @ARGV;
my ($fh, $time);
my (@arr1, @arr2);

my $use_dump = 0;


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 (1 + int (rand() * 10));
        if (rand() < 0.163) {
            $string = " Query${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;
my $s = do {local $/ = undef; <$fh>};
seek $fh, 0, 0;

print "\n\n";

$time = time;
my $match_count1 = 0;
my $i1 = 0;
my $xx;
while(<$fh>) {
  /^ ?Query/ && $match_count1 ++;
  push @arr1, $_;
  if ($use_dump and /^ Query/) {
        Dump $_;
        $i1 ++;
        last if $i1 > 5;
  }
}
printf "%f read lines from disk and do RE ($match_count1 matches).\n",
+ time - $time;

$fh->close;


open my $mfh, "<", \$s;
$time = time;
my $match_count2 = 0;

my $i2 = 0;
while(<$mfh>) {
  #  comment this out to avoid the OOM
  /^ ?Query/ && $match_count2++;

  #push @arr2, ($_ . "");  #  avoids OOM
  push @arr2, $_;          #  OOM!
  
  if ($use_dump and /^ Query/) {
    Dump $_;
    $i2++;
    last if $i2 > 5;
  }

}
printf "%f read lines from in-memory file and do RE ($match_count2 mat
+ches).\n", time - $time;

$mfh->close;
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