Don't turn off warnings, instead understand and fix the warnings.

Don't use prototype subs - they don't do what you think and will probably bite you.

If I understand what you want to do correctly the following should help:

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

while (<DATA>) {
    my @parts = split /(\|[^\|]*\|)/;
    
    for my $part (@parts) {
        if ($part =~ /\|(.*)\|/) {
            my @chars = $part =~ /([\da-f]+)/ig;
            
            $part = join '', map{chr(hex($_))} @chars;
        }
        
        print $part;
    }
}

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

Prints:

01234.dropbox.com
User-Agent:Google  Desktop
[download]