Rather than suck the entire file into memory (if it's bigger than the sample you posted), you could process it line-by-line and use the flip-flop operator (see perldoc perlop)

#!/usr/bin/perl                                                    
use strict;
use warnings;
use feature 'say';                                                 
    
my $HSSIN = shift or die "usage: $0 <inputfile";                   
open my $infile, $HSSIN or die "Can't open input file";            
    
my ($MSISDN, $cfu_found, $cfb_found);                              
    
while (<$infile>) {

    # this is true for all lines between these two markers 
    if (/^\s*<SUBBEGIN/.. /^<SUBEND/)  {                          

        if (/MSISDN=(.+)/){                                        
            $MSISDN = $1;                                          
        }
        if (/^\s*CF=CFU-TS10-ACT/){                                
            $cfu_found = 1;
        }
        if (/^\s*CF=CFB-TS10-ACT/){
            $cfb_found = 1;                                        
        }
    }
}   

if ($cfu_found and $cfb_found){
    say "cfu and cfb both found and MSISDN is $MSISDN";            
}
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