You can just use pack 'H*' on the string as follows:

    my $bin_str = pack 'H*', $hex_st;
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Or in the case of your example, something like this:

    ...
    if ( $pngData{$hashCode} ) {
        open    MyPNG, '>', $hashCode or die "Write Error: $hashCode: 
+$!\n";
        binmode MyPNG;
        print   MyPNG  pack 'H*', $pngData{$hashCode};
        close   MyPNG;
    }
    ...
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I often use this with qw() to keep the binary data nicely formatted:

    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
    );
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--
John.