But then Perl's Core "Encode" is not thread safe? :(

Suppose I split my program in two. First one creates 2 files, with single-byte encoded data and double-byte encoded data (and says "Equal", of course -- this image has less than 255 "objects").

use strict;
use warnings;
use 5.020;

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

my $img  = PDL::IO::Image-> new_from_file( 'test.png' );
my $pdl  = $img-> pixels_to_pdl-> short;

my $s1   = cc8compt( $pdl == 0 )-> byte;
my $s2   = cc8compt( $pdl == 0 );

say 'Equal' if decode( 'UTF16LE', ${ $s2-> get_dataref }) eq
                                  ${ $s1-> get_dataref };

writefraw( $s1, "fname-1" );
writefraw( $s2, "fname-2" );
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The 2nd program doesn't use PDL modules, it reads data from disk (and uses some hard-coded numbers):

use strict;
use warnings;
use 5.020;

use threads;

use Encode qw/ decode /;

say "No thread (byte)\n---------";
say 'Count: ', scalar @{ test( 1 )};

say "\nThread (byte)\n---------";
say 'Count: ', scalar @{ threads-> create( \&test, 1 )-> join };

say "\nNo thread (short)\n---------";
say 'Count: ', scalar @{ test( 2 )};

say "\nThread (short)\n---------";
say 'Count: ', scalar @{ threads-> create( \&test, 2 )-> join };

sub test {
    my $arg = shift;
    
    open my $fh, '<', "fname-$arg";
    binmode $fh;
    my $str = do { local $/; <$fh> };
    close $fh;
    $str = decode( 'UTF16LE', $str ) if $arg == 2;

    my ( $w, $h ) = ( 7616, 1200 );

    my @b = map {
        [ [ $w, 0 ], [ $h, 0 ] ]
    } 0 .. 145;

    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;
}
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The output:

No thread (byte)
---------
Time: 0
Count: 145

Thread (byte)
---------
Time: 1
Count: 145

No thread (short)
---------
Time: 1
Count: 145

Thread (short)
---------
Time: 51
Count: 145
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