Arguments are in the list @ARGV. So if you want to set $oggdir to the first argument, or to /media/tmp if there is no first argument, you could do something like:

my $oggdir;
if ($ARGV[0])
{
  $oggdir=$ARGV[0];
}
else
{
  $oggdir='/media/tmp';
}
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A shorter and more idiomatic way to do the same thing is:

my $oggdir = shift || '/media/tmp';
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shift removes the first item from a list and returns it; in the main body of a program (outside of subs), it defaults to using @ARGV. So this is a little different than the above; it will remove the first argument from @ARGV. If shift returns an argument, $oggdir will be set to it; otherwise it will use the other half of the or, and set it to the default value.

If you want more complex argument processing, try using Getopt::Std or Getopt::Long.