The function returns a formated string with 3 components joined with no token together. The components are 'packed' in to a string
OK so...

my $msg = pack("a".$maxMsgCodeLength."ia*",$msgCode, $port, $ip);
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"StartBE" is 7 characters long, this gets postfixed with spaces by
  $msgCode = $msgCode.(" " x ($maxMsgCodeLength-length($msgCode))); We'll say the max length is 10 here to make it easier.
"StartBE   " is now our string.

my $msg = pack( "a10ia*", $msgCode, $port, $ip );
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Can be written more clearly as

my $msg = pack( "a20 i a*", $msgCode, $port, $ip );
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From pack on perldoc:
a = A string with arbitrary binary data, will be null padded.
i = A signed integer value.

The three arguments ( $msgCode, $port, $ip ) get 'packed' into ( a10, i and a* ) respectively.

10 Number of characters in first arugment
* However many items are left

If you're familar with printf then it's simlar (but more powerfull then that )

printf( "%s %d %s", $msgCode, $port, $ip );
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You could use 'A' instead of 'a' in the pack statement. This will pad with spaces hence if you set A20 and only give a string length 10 then 10 spaces are added to the end rendering the line previous to pack useless. You'd probably want to make the function more generic...

print getMessage ( 'StartBE', $ip, $port ), "\n";

sub getMessage 
{
    my ( $msgCode, $ip, $port) = @_;
    return pack "A10 i A*", $msgCode, $port, $ip;
}
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J,