How can I modify received socket messages in Perl?

bonny95 has asked for the wisdom of the Perl Monks concerning the following question:

I am writing a Perl program that sends and receives messages from two sockets and acta as a switch. I have to modify the received messages received from one socket, prepend 3 bytes to the data, and finally send the modified messages to another socket. I adopt select()...sysread()...syswrite() mechanism to poll for messages between sockets. The received messages are stored in $buffer during modification.
Now I can use following way to get the received messages:

my $hexmsg = unpack("H*", $buffer);
my @msg = ( $hexmsg =~ m/../g );
[download]

then I can insert 3 bytes to @msg using splice(). However, I don't know how to pack the message in @msg into a scalar(such as $buffer) and send it to another socket by syswrite().
I know this method is fool and I believe there must be more clever ways. Can anybody help me with this problem? Thank you in advance!
BTW, are messages in $buffer binary?

Comment on How can I modify received socket messages in Perl? Download Code

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Re: How can I modify received socket messages in Perl? by ikegami (Patriarch) on Mar 09, 2009 at 15:15 UTC
First, I doubt your `$buffer` contains a message, except by luck. But if it does, why not just do `$buffer = "\x{12}\x{34}\x{56}" . $buffer;` [download] or equivalent `$buffer = (map chr, 0x12, 0x34, 0x56) . $buffer;` [download] BTW, are messages in $buffer binary? Assuming `$buffer` is being populated by `sysread`, `$buffer` is a string where each character represents a byte.	[reply] [d/l] [select]
Re^2: How can I modify received socket messages in Perl? by bonny95 (Initiate) on Mar 10, 2009 at 00:57 UTC
I am sorry that I didn't tell things clearly. I just want to insert 3 bytes into received msg between the header and body, but not in the front or end. Can substr() work?	[reply]
Re^3: How can I modify received socket messages in Perl? by ikegami (Patriarch) on Mar 10, 2009 at 02:52 UTC
You don't have a header and body, just a bunch of bytes. First things first, you need to identify the header and the body.	[reply]
Re^4: How can I modify received socket messages in Perl? by bonny95 (Initiate) on Mar 10, 2009 at 04:43 UTC
Re^5: How can I modify received socket messages in Perl? by ikegami (Patriarch) on Mar 10, 2009 at 05:18 UTC
Re: How can I modify received socket messages in Perl? by Anonymous Monk on Mar 09, 2009 at 15:14 UTC
then I can insert 3 bytes to @msg using splice(). However, I don't know how to pack the message in @msg into a scalar(such as $buffer) and send it to another socket by syswrite(). `$bytes = "\0\0\0"; $buffer = $bytes . $buffer;` [download] syswrite	[reply] [d/l]