I think this covers all the cases:
my $num = shift;
my $firstlen = int(length($num)/2 + .5);
my $backlen = length($num) - $firstlen;
my $first = substr($num, 0, $firstlen);
my $closest = 0;
for (1..2) {
my $new = $first . substr(reverse($first), -$backlen);
$closest = $new if abs($new - $num) < abs($closest - $num);
$first += $num <=> $closest;
}
print "Closest palindrome to $num is $closest\n";
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