I re-wrote it so that it was a little more readable, but I still haven't a clue what it does. What is $self->{S_KEY}? What does it contain? What is $shift? Can you document the function's inputs and what you expect to get out of it? Some sample data might be helpful.

sub Make_shiftedKey ($) {
    my $self  = shift;
    my $shift = shift;

    if ($shift % 2) {
        $self->{S_KEY} = substr($self->{S_KEY}, 1, length($self->{S_KE
+Y}));
    }
    else {
        $self->{S_KEY} = substr($self->{S_KEY}, 0, length($self->{S_KE
+Y}) - 1);
    }

    for (my $i=0; $i <= ($shift - 1) % length $self->{S_KEY}; $i++) {
        my $key = substr($self->{S_KEY}, 0, 1);
        $self->{S_KEY} = substr($self->{S_KEY}, 1, length($self->{S_KE
+Y}) - 1);
        $self->{S_KEY} .= $key;
    }

    return $self->{S_KEY};
}
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