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pseudo code
check line for tabMSISDN=digits;
if found extract digits and assign to line 
otherwise assign MSISDN=notSet to the line for printing later

if (/^(\t*MSISDN=(\d+));/) {   
        
...
        $line = "<$2>";   #group 2
        #otherwise assign MSISDN=notSet to the line for printing later
}

 #!/usr/bin/perl 
use strict;
...
print OUTFILE "Update Command $line\n";
close INFILE;
close OUTFILE;

<SUBBEGIN
    MSISDN=123476789678;
...
    ODBIC=BIC;
    ODBOC=BAOC;
<SUBEND

desired output 

Update Command <123476789678>,BIC,BAOC
Update Command MSISDN=notSet,BIC, BAOC