This works as specified. No need to nest the blocks.

print "1. Do you want to enter value for Starting account balance. (y/
+n)?\n";
chomp(my $answer = <STDIN>);

my $SAB;
if ('y' eq $answer) {
    print 'Please enter SAB=';
    $SAB = <STDIN>;
}

print "2. Do you want to enter the Date (y/n)?\n";
chomp($answer = <STDIN>);
exit if 'n' eq $answer;
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However, you have not specified what to do if the user entered something else.