I figured you had a version that actually did work. FWIW, this is the only thing I could come up with that is as fast as the substitution. Unfortunately, it slows down when you put in the check for legit hex values (and it is much less readable). 
sub noregex {
    local $_ = $str;

    my $pos = 0;
    while ( (my $idx = index($_, '%', $pos)) > -1) {
        $pos = $idx + 1;
        my $code = substr($_, $pos, 2);
        substr $_, $idx, 3, pack("C*", hex $code);
    }
    return $_;
}
__END__

Benchmark: timing 100000 iterations of NO_REGEX, REGEX...
  NO_REGEX:  8 wallclock secs ( 8.77 usr +  0.00 sys =  8.77 CPU) @ 11
+402.51/s (n=100000)
     REGEX:  9 wallclock secs ( 9.15 usr +  0.00 sys =  9.15 CPU) @ 10
+928.96/s (n=100000)

[download]

In reply to RE: RE: RE: RE: form parsing, hex, HTML formatting by takshaka
in thread form parsing, hex, HTML formatting 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.