Protocol::Modbus is the module that you want to use. The documentation is very sparse, but the source has the info that you need. I gave it quick look and managed to get this going. I used the generic approach.
#!/usr/bin/perl -l
use strict;
use warnings;
$|=1;
my $proto = Protocol::Modbus->new( driver => 'TCP', transport => 'TCP
+' );
my $request1 = $proto->request(
function => 1,
address => 1,
quantity => 1,
);
my $request2 = $proto->request(
function => 2,
address => 1,
quantity => 1,
);
my $request3 = $proto->request(
function => 3,
address => 1,
uantity => 1,
);
my $request4 = $proto->request(
function => 4,
address => 1,
quantity => 1,
);
print "=" x 60;
print "Using unpack: \n";
print unpack ('H*', $request1->pdu());
print unpack ('H*', $request2->pdu());
print unpack ('H*', $request3->pdu());
print unpack ('H*', $request4->pdu());
print "=" x 60;
print "Using Pretty-Print: \n";
print $request1;
print $request2;
print $request3;
print $request4;
print "=" x 60;
print "stringify: ";
print $request1->stringify();
print $request2->stringify();
print $request3->stringify();
print $request4->stringify();
print "=" x 60;