Your bug is that $SubRef is interpolated into the eval. Oops. But you don't need to do that: 
sub Half
{
    my $n = shift;
    my $SubBody = Power_Of_Two( $n )    ? 
        "return \$_[$[] >> Log2( $n );" :
        "return int( \$_[$[] / $n );";
    my $code = "sub { $SubBody }";
    my $sub = eval $code;
    die "Code '$code' gave error $@" if $@;
    return $sub;
}

[download]
But given the overhead of entering an anonymous function it is not worth looking for optimizations this way. Instead stop and think about "optional hooks" in your code. Where closures rock is as a way to provide optional hooks when you compile a function without paying any run-time price if the hook was not included. So instead of doing run-time checks for whether debugging (for instance) is on, make the debugging checks be inserted or not when you create the string to eval for the sub.

As the rule goes, optimize on the level of big design decisions, not faster local constructs.

In reply to RE (tilly) 1: Fly Subroutines on the Fly by tilly
in thread Fly Subroutines on the Fly by Adam

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.