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";

[download]

prints: 

common1: c b
common2: c b

[download]

In reply to Re^3: common keys in two hashes by toolic
in thread common keys in two hashes by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.