>>91 Perl5 >>92 よりもう少しシンプルに

use List::Util min;
for ('1,1,1,1,2,2,2,3,3,4', '1,2,3,4,5,5,4,3,2,1', '3,1,4,1,5,9,2,6,5,3,5') {
　my %h;
　$h{$_}++ for split ',';
　$s = min grep{1 == $h{$_}} keys %h;
　$s //= -1;
　print "$_ -> $s\n";
}

実行結果
~ $ perl 15_91.pl
1,1,1,1,2,2,2,3,3,4 -> 4
1,2,3,4,5,5,4,3,2,1 -> -1
3,1,4,1,5,9,2,6,5,3,5 -> 2