The question is related to PDL: Looking for efficient way to extract sub-images, by finding bounding boxes of "objects". But I don't think it has anything to do with PDL, images, etc. The $str is just a very long string. Using $1 instead of $& doesn't help.

Perl is 5.20.2 Win32 Strawberry, or 5.24 Win64.

use strict;
use warnings;
use 5.020;

use threads;

use PDL;
use PDL::IO::Image;
use PDL::Image2D;
use Encode qw/ decode /;

PDL::no_clone_skip_warning;

say "No thread\n---------";
say 'Count: ', scalar @{ test( 'test.png' )};

say "\nThread\n---------";
say 'Count: ', scalar @{ threads-> create( \&test, 'test.png' )-> join
+ };

sub test {
    my $fn  = shift;
    
    my $img = PDL::IO::Image-> new_from_file( $fn );
    my $pdl = $img-> pixels_to_pdl-> short;
    my $s   = cc8compt( $pdl == 0 );
    my $str = decode 'UTF16LE', ${ $s-> get_dataref };

    my ( $w, $h ) = $s-> dims;

    my @b = map {
        [ [ $w, 0 ], [ $h, 0 ] ]
    } 0 .. $s-> max;

    my $t = time;
    
    for my $y ( 0 .. $h - 1 ) {
        my $s = substr( $str, $y * $w, $w );
        while( $s =~ m[[^\0]+]g ) {
            my $c = ord( $& );
            $b[ $c ][ 0 ][ 0 ] = $-[0]     if $-[0]     < $b[ $c ][ 0 
+][ 0 ];
            $b[ $c ][ 0 ][ 1 ] = $+[0] - 1 if $+[0] - 1 > $b[ $c ][ 0 
+][ 1 ];
            $b[ $c ][ 1 ][ 0 ] = $y        if $y        < $b[ $c ][ 1 
+][ 0 ];
            $b[ $c ][ 1 ][ 1 ] = $y        if $y        > $b[ $c ][ 1 
+][ 1 ];
        }
    }
    say 'Time: ', time - $t;

    shift @b;
    return \@b;
}

[download]

The output:

No thread
---------
Time: 2
Count: 145

Thread
---------
Time: 52
Count: 145

[download]

In reply to Why this code is so slow if run in thread? by vr

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