all values for the keys of %set2 should be set to undef

They are undef, try this:

my %hash2;                  
undef @hash2{qw(a b c)};
my %hash1;
undef @hash1{qw(c d e)};
undef @hash2{keys %hash1};
print "$_ ",defined($hash2{$_})?"def\n":"undef\n" for keys %hash2;

[download]
This technique works great when you want undefined don't care about the values. If you want to preserve defined values, then you have to do something like what Zaxo has.

Update: corrected by mirod, anyway, this works great when working with sets and you don't care about the values.

In reply to Re: Re: Re: Re: Fast Way to Combine Two Hashes by runrig
in thread Fast Way to Combine Two Hashes by arunhorne

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.