Note that %combined = (%hash1, %hash2); suffers from performance penalty. If performance's a concern, it's always better to do map { $hash1{$_} = $hash2{$_} } keys %hash2; (unless you want to keep %hash1 and %hash2 intact, which doesn't seem to be the case in your question)

use Benchmark;

my $size = 100000;
my %hash1 = map { $_ => 1 } (0..$size);
my %hash2 = map { $_ => 1 } ($size..(2*$size));
my $t0 = new Benchmark;
my %combined = (%hash1, %hash2);
my $t1 = new Benchmark;
print "Combining took:",timestr(timediff($t1, $t0)),"\n";
$t0 = new Benchmark;
map { $hash1{$_} = $hash2{$_} } keys %hash2;
my $t1 = new Benchmark;
print "Map took:",timestr(timediff($t1, $t0)),"\n";
[download]

When size = 100000 and 1000000 respectively, the results:
inq123@perlmonks$ perl test.pl
Combining took: 0 wallclock secs ( 0.28 usr +  0.02 sys =  0.30 CPU)
Map took: 0 wallclock secs ( 0.18 usr +  0.00 sys =  0.18 CPU)
inq123@perlmonks$ perl test.pl
Combining took:42 wallclock secs (41.53 usr +  0.21 sys = 41.74 CPU)
Map took: 2 wallclock secs ( 1.89 usr +  0.05 sys =  1.94 CPU)

So performance penalty is manifested when the hashs contain tens of thousands of elements, which is not too rare.

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