Alright, here it goes:

$_='Just';
$_='"...erxpnU;yerC;erugban;gfhW"gavec';
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Easy to see, just setting $_ to something, then resetting it.

$-='another Perl';
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Useless; $- is the number of lines left on the page of the currently selected output channel, and cannot be a string value (or a negative value). $- remains zero after this asignment.

y/\\abcdefghijklmnopqrstuvwxyz ;
/'nopqrstuvwxyzabcdefghijklm                    /;
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A simple translation on $_: Now the value is almost the reverse of "print "Just another Perl hacker..."", we just need one more translation to get it there:

$-='Hacker'; #again, this is just filler
y/!!ABCDEFGHIJKLMNOPQRSTUVWXYZ       ;
/''NOPQRSTUVWXYZABCDEFGHIJKLM /;  #here's that translation I spoke of
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Now, the string is ready to be eval'ed

eval reverse  $_;
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Again, this is just filler, it evaluates to a string, which is a true value, but does nothing.

''.''.''.''.''.'print "Just another Perl Hacker"';
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The 15 year old, freshman programmer,
Stephen Rawls