sorry wrong code

update : i appologize por posting accidently into the wrong window

well you were almost there. here is a version utilizing your approach:

use strict;
use Data::Dumper;


%muthash = (
        key1 => '1',
        key15 => '3',
        key150 => '1', );
%copyhash =(
        key1 => '1',
        key15 => '1',
        key140 => '1', );
%mergedhash = ();
%mergedhash = map {$_=> $muthash{$_} + $copyhash{$_}; } (keys %muthash
+, keys %copyhash);
print Dumper(\%mergedhash);
[download]

however, if you are biginner, do not go exploring functions like map until you master "for", "while" and "foreach" loop. Thus equivalent to your proposed solution is :

my %muthash = (
        "key1" => 1,
        "key15" => 3,
        "key150" => 1 );
my %copyhash = (
        "key1" => 1,
        "key15" => 1,
        "key140" => 1 );
my %mergedhash;
foreach (keys %muthash){
$mergedhash{$_} = $muthash{$_}+ $copyhash{$_};

}
foreach (keys %copyhash){
$mergedhash{$_} = $muthash{$_}+ $copyhash{$_} unless defined $mergedha
+sh{$_};
}
print Dumper(\%mergedhash);
[download]

cheers