my $bin_str = pack 'H*', $hex_st;


##</code><code>##

    ...
    if ( $pngData{$hashCode} ) {
        open    MyPNG, '>', $hashCode or die "Write Error: $hashCode: $!\n";
        binmode MyPNG;
        print   MyPNG  pack 'H*', $pngData{$hashCode};
        close   MyPNG;
    }
    ...



##</code><code>##


    my $bin_str = pack 'H*', join '', qw(
        18 00 1B 00 21 00 00 01 0B 00 00 00 01 00 00 00
        21 00 00 01 0B 00 00 00 01 00 00 00 18 00 1B 00
        00 00 00 0D 3B 00 00 00 00 04 00 00 00 02 00 FF
        00 01 0B 00 00 00 01 00 00 00
    );