Just to be perverse, here is exactly what you asked for

print rx_reverse( shift );
sub rx_reverse {
    local $_ = shift;
    my $length = length;

    for my $offset ( 0 .. int($length / 2) ) {
        my $rx = rx_gen( $length, $offset );
        s/$rx/$1$4$3$2/;
    }
    $_
}

sub rx_gen {
    my $length = shift;
    my $offset = shift;
    my $skip = $length - 2 * ($offset + 1);
    return unless $length > 0 and
                  $offset <= ($length / 2) and
                  $skip >= 0;

    $offset = $offset ? "((?s:.){$offset})" : '()';
    $skip   = $skip   ? "((?s:.){$skip})"   : '()';
    return "^$offset((?s:.))$skip((?s:.))"
}
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