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

##</code><code>##

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

##</code><code>##

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


my $HSSIN='D:\testproject\sample-input.txt';
my $ofile = 'D:\testproject\sample-output.txt';
my $add;
open (INFILE, $HSSIN) or die "Cant open input file";
open (OUTFILE,"> $ofile" ) or die "Cant open file";

my $line;
while (<INFILE>) 
{
	if (/^(\t*MSISDN=(\d+));/) {    
        
        print OUTFILE "Update Command $line\n" if defined $line;  
        $line = "<$2>";   #group 2
        
    }
	
    	if (/(\t*ODBIC=([\w]+?\w.*));/)
    	{  
        	 #print OUTFILE "$line\n" if defined $line;   
        	 #$line = $2;
        	 $add = $2;
    		 $line .=  ",$add"; 
        	
        }
        
        
        
        if (/(\t*ODBOC=([\w]+?\w.*));/)	
        {
        		$add = $2;
    			$line .=  ",$add";
        }	
        
        
          
}

print OUTFILE "Update Command $line\n";
close INFILE;
close OUTFILE;

##</code><code>##

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

##</code><code>##

desired output 

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