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    <p>Step № 1 to understand obfuscated Perl: run it through <code>-MO=Deparse</code>. Then, we get this output: (with a small syntax error removed)</p> <pre><code>use warnings; use strict; my($japh, $q, $qq, %b) = "Just another Perl hacker\n"; $_ = join('', (1045737470, 21650696, 478656270, 205562730, 602861636, 965627100)); s/9/f/; s/9/b/; $q = $_; *9 = sub { $japh = ''; print $japh; } ; map {m{^((?i)(?#q#; print \$japh;#()[^ for (my $(b) = 1; $(b) &lt; $(q); \$b++;) { /g])+[\](^.^)\[ } ]*$}x ? do { $qq = ord($_) - 96 . (~~%b - $?); $/ = $+; $q =~ s[$qq][$/]g } : 9-&gt;({});} 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'; $qq = [$q, %b]; $\ = pack('h*', '' . $$qq[0]); 9-&gt;(); </code></pre> <p>Still not pretty, but better. Especially, this regex looks interesting:</p> <pre><code>m{^((?i)(?#q#; print \$japh;#()[^ for (my $(b) = 1; $(b) &lt; $(q); \$b++;) { /g])+[\](^.^)\[ } ]*$}x </code></pre> <p>The <code>(?# ... )</code> is an embedded comment, which we can remove. Next on are the character classes</p> <pre><code>[^ for (my $(b) = 1; $(b) &lt; $(q); \$b++;) { /g] </code></pre> <p>and</p> <pre><code>[\](^.^)\[ } ] </code></pre> <p>Here, they are equivalent to <code>[^bfgmoqry]</code> and <code>[\[\]()^.}\s]</code>, but as the regex will only ever be matched against single chars, the <code>[...]*</code> will match zero characters.</p> <p>Thus, the regex is equivalent to</p> <pre><code>m/^([^bfgmoqry])$/ </code></pre> <p>in this <em>special case</em> (being matched against <code>"a" .. "z"</code>).</p> <p>The <code>*9 = sub {...}</code> assigns a coderef to a glob. Because <code>9</code> isn't a valid subroutine name, but can still be used with the <code>-&gt;</code> operator, <code>9-&gt;()</code> invokes that coderef. This is just like with <code>sub foo {...}</code>, <code>foo()</code> and <code>"foo"-&gt;()</code> are the same thing.</p> <p>The char <code>a</code> is encoded as decimal <code>97</code> as ASCII, so <code>ord("a")-96</code> is <code>1</code>, for <code>z</code>, that would be 26.</p> <p>The <code>$?</code> is the last child error, and should be zero. <code>$+</code> is the value of the last capture group (ergo, <code>$_</code>). The <code>~~%b</code> double-negates an empty hash. The scalarification of an empty hash is <code>0</code>, so double bitwise negation produces zero again.</p> <p>The <code>$/ = $+; $q =~ s[$qq][$/]g</code> is just a bit obfuscation; as <code>$+</code> is <code>$_</code>, we could directly write <code>$q =~ s/$qq/$_/g</code>.</p> <p>The <code>$qq = [$q, %b]; ... $$qq[0]</code> is lame, a <code>$\ = pack('h*', $q)</code> suffices.</p> <p>In the <code>9</code> sub, the empty string will be printed, followed by <code>$\</code>, which is set to the empty string up to the penultimate line. Removing this level of indirection, and any now unused variables, as well as the simple substitutions, we get:</p> <pre><code>use warnings; use strict; my $q = '1045737470216506f6478656270205562730602861636b65627100'; for ("a" .. "z") { m/^([^bfgmoqry])$/ or next; my $qq = ord($_) - 96; $q =~ s/${qq}0/$_/g; } print pack('h*', $q); </code></pre> <p>The <code>s/${qq}0/$_/g</code> substitution can only match at zeros, and we know that the output will be some hex-number for the <code>pack</code>. Possible positions for matching are:</p> <pre><code>1045737470216506f6478656270205562730602861636b65627100 *^ *^ *^ *^*^ *^*^ *^^ a g e g b c f a </code></pre> <p>of which <code>b, f, g</code> is forbidden due to the regex. The resulting string is</p> <pre><code>a45737470216e6f64786562702055627c602861636b65627a0 </code></pre> <p>Which decodes to the JAPH.</p>
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