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use strict;
use DBI;
...
for my $index (0..$#left_column) {
    $sth->execute($right_column, $left_column);
};

my $statement = 'UPDATE wordtable SET right_column = CASE ';
$statement .= join "\n", map { "WHEN left_column = '$left_column[_]' T
+HEN '$right_column[$_]'" } 0..$#left_column;
$statement .= "END;";
print $statement;