It will not work, because you want to test, which sub is faster with one iteration. In the documentation of Test::Benchmark it is recommended to use at least 10000 iterations. try this one (due to the Test::Benchmark-docu):

#!/usr/bin/perl
use Test::More tests => 1;
use Test::Benchmark;
is_faster(10000,sub {$a=9;
        print "enter a name \n";
        $b=<STDIN>;
        print "$b\n";},
       sub { print "give input";
 $b=<STDIN>;
 chomp($b) ;
 print "$b \n";},"res1 is faster than res2");
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Name "main::a" used only once: possible typo at bench.pl line 6.
You just define and initialise this variable so perl givs you a warning!