sub JDP::longest_palindrome # operates on $_
{
    my $pal;
    for my $i ( 0 .. length($_) )
    {
        last if defined($pal) && length($_)-$i < length($pal);
        my $j = rindex $_, substr( $_, $i, 1 );
        while ( $j > $i )
        {
            my $s = substr $_, $i, $j-$i+1;
            if ( $s eq reverse $s ) # it's a palindrome
            {    # but is it the longest yet found?
                $pal = $s unless defined $pal && length($pal) > length
+($s);
            }
            $j--;
            $j = rindex $_, substr( $_, $i, 1 ), $j;
        }
    }
    $pal
}
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