'@$msg' dereference the array reference '$msg'.
'join ("\n", @$msg )' is used for join the '$msg' array reference elements into a string separated by "\n". For eg.

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

my $msg = ['apple', 'banana', 'orange']; # array reference
my @msg = @$msg;             # dereferncing the arreay ref

print "Dereferenced: @msg\n";

print join("\n",@$msg);                        # prints the array elem
+ents sperated by "\n".
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Please go through perlref and join for more information.