use strict;   
use warnings;
use List::Util qw(max);
use Data::Dumper;

my %h = (a=>2, b=>8, c=>7);
my $max = max(values %h);
for (keys %h) {
    delete $h{$_} unless $h{$_} == $max
}
print Dumper(\%h);

__END__
$VAR1 = {
          'b' => 8
        };