the program I suggested does not set MSISDN=0, it does initialize the beginning of any new output line being built to the string "MSISDN=0". Then, if a line with a MSISDN is found in the input data, the new line beginning is changed to the value found for the MSISDN in the input record; if no MSISDN is found in the input, the initial value is kept and the output line will start with the string "MSISDN=0", which is what you want when there is no MSISDN line in the input.

the program ... does not set MSISDN=0, it does initialize the beginning of any new output line being built ...

I haven't been following the slow, tantalizing revelation of the specification of the problem closely enough to know if this is even a possibility, but the code here seems to fail when the first <SUBBEGIN "record" does not contain an MSISDN=n; field, e.g.:

__DATA__

<BEGINFILE>
<SUBBEGIN
     IMSI=232191400010339;
     CF=CFU-ALL-PROV-NONE-YES-NO-NONE-YES-65535-NO-NO-NO-NO-NO-NO-NO-N
+O-NO-NO;
     CF=CFB-ALL-PROV-NONE-YES-YES-NONE-YES-65535-NO-NO-NO-NO-NO-NO-NO-
+NO-NO-NO;
     CF=CFNRY-ALL-PROV-NONE-YES-YES-NONE-YES-65535-NO-NO-NO-NO-NO-NO-N
+O-NO-NO-NO;
     CF=CFNRC-ALL-PROV-NONE-YES-NO-NONE-YES-65535-NO-NO-NO-NO-NO-NO-NO
+-NO-NO-NO;
     CF=CFD-TS10-ACT-91436903000-YES-YES-25-YES-65535-YES-YES-NO-NO-NO
+-YES-YES-YES-YES-NO;
<SUBEND
<SUBBEGIN
     IMSI=232191400010332;
     MSISDN=436906901235;
     CF=CFU-ALL-PROV-NONE-YES-NO-NONE-YES-65535-NO-NO-NO-NO-NO-NO-NO-N
+O-NO-NO;
     CF=CFB-ALL-PROV-NONE-YES-YES-NONE-YES-65535-NO-NO-NO-NO-NO-NO-NO-
+NO-NO-NO;
     CF=CFNRY-ALL-PROV-NONE-YES-YES-NONE-YES-65535-NO-NO-NO-NO-NO-NO-N
+O-NO-NO-NO;
     CF=CFNRC-ALL-PROV-NONE-YES-NO-NONE-YES-65535-NO-NO-NO-NO-NO-NO-NO
+-NO-NO-NO;
     CF=CFD-TS10-ACT-91436903000-YES-YES-25-YES-65535-YES-YES-NO-NO-NO
+-YES-YES-YES-YES-NO;
<SUBEND
<BEGINFILE>
<SUBBEGIN
     IMSI=232191400010339;
     MSISDN=436906901231;
     CF=CFU-ALL-PROV-NONE-YES-NO-NONE-YES-65535-NO-NO-NO-NO-NO-NO-NO-N
+O-NO-NO;
     CF=CFB-ALL-PROV-NONE-YES-YES-NONE-YES-65535-NO-NO-NO-NO-NO-NO-NO-
+NO-NO-NO;
     CF=CFNRY-ALL-PROV-NONE-YES-YES-NONE-YES-65535-NO-NO-NO-NO-NO-NO-N
+O-NO-NO-NO;
     CF=CFNRC-ALL-PROV-NONE-YES-NO-NONE-YES-65535-NO-NO-NO-NO-NO-NO-NO
+-NO-NO-NO;
     CF=CFD-TS10-ACT-91436903000-YES-YES-25-YES-65535-YES-YES-NO-NO-NO
+-YES-YES-YES-YES-NO;
<SUBEND
[download]

which produces the output:

c:\@Work\Perl\monks\gbwien>perl parse_SUBBEGIN_recs_1.pl
,CFU-ALL-PROV-1/1/1/0,CFB-ALL-PROV-1/1/1/0,CFNRY-ALL-PROV-1/1/1/0,CFNR
+C-ALL-PROV-1/1/1/0,CFD-TS10-ACT-1/1/1/436903000
436906901235,CFU-ALL-PROV-1/1/1/0,CFB-ALL-PROV-1/1/1/0,CFNRY-ALL-PROV-
+1/1/1/0,CFNRC-ALL-PROV-1/1/1/0,CFD-TS10-ACT-1/1/1/436903000
436906901231,CFU-ALL-PROV-1/1/1/0,CFB-ALL-PROV-1/1/1/0,CFNRY-ALL-PROV-
+1/1/1/0,CFNRC-ALL-PROV-1/1/1/0,CFD-TS10-ACT-1/1/1/436903000
[download]

If this configuration of data is even possible, it seems easy enough to fix by just changing the
    my $line;
statement just before the  while (<DATA>) { ... } loop to
    my $line = "MSISDN=0";


(Sorry for any wraparound artifacts!)

<tangent>
The use of  <SUBBEGIN and  <SUBEND markers in the data makes me wonder if gbwien is working on a project for the Church of the SubGenius.
</tangent>

---

Give a man a fish:  <%-{-{-{-<

Yes, AnomalousMonk, you're right.

The $line variable should be initialized at the time of declaration for the script to work correctly in the event that the first record block does not have an MSISDN line.

Thanks for the correction.