I just tried the code you posted. But I'm getting the same result like with my code. The loop is somehow not entered as well: 
c:\Dropbox\Scripts>perl PortTest2.pl

Enter AT command:ATI
Sending: 'ATI'
reading...

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From my understanding it should at least print the buffer size one time if there is 1 byte of data. The AT-command (ATI) I'm trying should give me something like this: 
ATI
Manufacturer: Company Name
Model: 1234
Revision: AABBCCDDEE-123456  1  [Dec 01 2012 14:00:00]
IMEI: 123456789012345
+GCAP: +CGSM,+DS,+ES

OK

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In reply to Re^4: Trying to read from serial port - not getting a response by adiuva
in thread Trying to read from serial port - not getting a response by adiuva

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