It may be that on my first reading of your node I understood it backwards, in which case the following code is more likely useful:

#!/usr/bin/perl
use strict;
use warnings;

while (<DATA>) {
    chomp;

    my @parts = split /(\|[^\|]*\|)/;
    
    for my $part (@parts) {
        if ($part =~ /\|(.*)\|/) {
            $part = join '', $part =~ /([\da-f]+)/ig;
        } else {
            $part = join '', map {sprintf '%02x', ord $_} split '', $p
+art;
        }
        
        print $part;
    }
    
    print "\n";
}

__DATA__
|30 31 32 33 34|.dropbox.com
User-Agent|3A|Google|20 20|Desktop
[download]

Prints:

30313233342e64726f70626f782e636f6d
557365722d4167656e743A476f6f676c6520204465736b746f70
[download]

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