When writing a script for command-line use, I try to make it easy for the user to get enough information about the expected / allowed args and options. Given your very limited example, I would tend to do it like this:

my $usage = "Usage: $0 [path]\n default: value of env.variable LOGS_EV
+\n";

my $destdir;
if ( @ARGV ) {
    $destdir = $ARGV[0];
}
elsif ( $ENV{'LOGS_EV'} ) {
    $destdir = $ENV{'LOGS_EV'}
}
else {
    warn "Please supply a path or set LOGS_EV\n\n";
    die $usage;
}
[download]