my ($major,$minor) = get_my_sql_version();
my $statement;
if ($major < 5) {
    $statement = 'show variables';
} elsif ("$major.$minor" eq "5.0") {
    $statement = 'show global variables';
} elsif ("$major.$minor" gt "5.0") {
    $statement = 'Select * from information_schema.global_variables order by 1';
};

$dbh->execute($statement);
...