I've managed to reduce my confusion to a very simple example: 

sub two_hard { @_ = @_[0,1]; @_ }
sub two_easy {      @_[0,1]     }

print "two easy: " . two_easy( 'one', 'two' ) . "\n";
print "two hard: " . two_hard( 'one', 'two' ) . "\n";

[download]

I think these two should be equivalent. They're both returning the same thing, but one of them does it without assigning it to something else first. (This question comes from trying to optimize some code that is like two_hard into something like two_easy.) However, in spite of their obvious similarity, the output of the above is: 

two easy: two
two hard: 2

[download]

Why does this happen?

In reply to Why does assignment change the result? 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.