Experimenting on your own will prove that only one search is needed. You did try this out on your own, didn't you? Anyway, here is a simple example where the common keys are b and c:

#!/usr/bin/env perl

use warnings;
use strict;

my %hash1 = (a=>1,b=>2,c=>3,d=>4);
my %hash2 = (     b=>5,c=>6,    e=>7);
my @common = ();
foreach (keys %hash1) {
    push(@common, $_) if exists $hash2{$_};
}
# @common now contains common keys
print "common1: @common\n";

@common = (); # empty the array
# reverse the search:
foreach (keys %hash2) {
    push(@common, $_) if exists $hash1{$_};
}
print "common2: @common\n";
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prints:

common1: c b
common2: c b
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