use strict;
use warnings;

use XML::Simple;

my $config = XMLin('foo.xml', ForceArray => [qw(tab button)]);

my @tabs = @{$config->{notebook_tabs}{tab}};

my $n = 1;
for my $t (@tabs) {
    print "In tab No. $n\n";
    if (exists $t->{button}) {
        print "\t found ", scalar(@{$t->{button}}), " button\n";
    } else {
        print "\t no tabs\n";

    }

    $n++;
}
__END__
In tab No. 1
         found 2 button
In tab No. 2
         no tabs