use strict ;
use warnings ;
use Data::Dumper ;

my %hash_origin = ( 'key1' => 1, 'key2' => 1, 'key3' => 1 ) ;
my @keys = qw( key1 key1 key2 key2 key2 ) ;
my %hash ;

foreach ( @keys ) {
	my $key = $_ ;
	$hash{ $key } += $hash_origin{ $key } ;
}
print Dumper( \%hash ) ;
__END__
$VAR1 = {
          'key1' => 2,
          'key2' => 3
        };