I don't think this is related to my previous Substitution unexpectedly very slow, in Strawberry, which GH ticket is marked as fixed. I can't check this new issue with 5.39. But if the fix also cures what's described below -- then fine. + Preliminary check with online Perl doesn't show anomalies, so looks like Linux is OK, it's Windows issue.

The recent File::Slurp discussion made me have a look at how it behaves in list context. In particular, a look at regex it uses to split, at line #133. I wondered if it's really the best method, then ran some tests. Accidentally, in the process I stumbled upon really weird results. As frustrating as it is, but close to unsellability, kind of.

I did "physical file IO", not "scalar IO", to better demonstrate how it can affect (and always did?) the very basic daily operations. "Preheat" immediately disposes of allocated array, it's not well-known "pre-set $#array or hash size for better performance".

use strict;
use warnings;
use feature 'say';
use Time::HiRes 'time';

if ( !@ARGV ) {
    say $^V;
    for my $size ( 5e5, 1e6, 5e6 ) {
        say "Array size: $size";
        for my $method ( 0, 1, 2 ) {
            for my $heat ( 0, 1 ) {
                system $^X, $0, $method, $heat, $size
            }
        }
    }
}
else {
    my ( $method, $preheat, $size ) = @ARGV;

    my $s = join ' ', 0 .. 9;
    $s .= "\n";
    $s x= $size;
    chomp $s;
    
    my $t = time;
    
    if ( $preheat ) {
        my @garbage = ( undef ) x $size
    }
    
    my @a;
    if ( $method == 0 ) {               # split
        @a = split /(?<=\n)/, $s
    }
    elsif ( $method == 1 ) {            # global match
        @a = $s =~ /(.*?\n|.+)/gs
    }
    elsif ( $method == 2 ) {            # IO (list context)
        open my $fh, '>', 'garbage.tmp';
        binmode $fh;
        print $fh $s;
        close $fh;
        open $fh, '<', 'garbage.tmp';
        binmode $fh;
        @a = <$fh>;
        close $fh;
    }
    
    printf "\t%s, %s:\t%.3f\n", 
        ( <split match io(list)> )[ $method ],
        ( $preheat ? 'pre-heat' : 'no pre-heat' ),
        time - $t
}

[download]

Result:

v5.38.0
Array size: 500000
        split, no pre-heat:     0.627
        split, pre-heat:        0.631
        match, no pre-heat:     0.855
        match, pre-heat:        0.292
        io(list), no pre-heat:  0.893
        io(list), pre-heat:     0.274
Array size: 1000000
        split, no pre-heat:     1.272
        split, pre-heat:        1.286
        match, no pre-heat:     3.604
        match, pre-heat:        0.583
        io(list), no pre-heat:  3.498
        io(list), pre-heat:     0.556
Array size: 5000000
        split, no pre-heat:     6.356
        split, pre-heat:        6.346
        match, no pre-heat:     79.586
        match, pre-heat:        2.885
        io(list), no pre-heat:  84.150
        io(list), pre-heat:     2.744

[download]

In reply to Strawberry: Both IO and global match are VERY SLOW. Unless pre-heated (but why?) by Anonymous Monk

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