Very nice, you've got them all scoped as lexicals. None of the following is criticism, you've chosen a style which is very good for maintainance.

There are a few things you could do to limit scopes further. The simplest and least obtrusive would be a bare block around my $yesterday' and my @msg; like this:

use LWP::UserAgent;
{
    print "\n  Fetching log...\n\n";
    my $yesterday = Yesterday();
    my @msg = Main(
      'joe@modem.com',           # remote domain
      'yosef',                   # remote id
      'secret',                  # remote password
      "access.log.$yesterday",   # remote logfile input
      'c:/logs',                 # local dir to write to
      'access.log',              # local logfile output
    );
    print "$_\n" for(@msg);
}
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I included the initial print statement in the block, just in case you want to localize $\ and kin.

My own inclination is to do away with named variables as much as is consistent with maintainability and readability, so I might have written that as:

{
    local $\="\n";
    print "\n  Fetching log...\n";
    print for Main(
              'joe@modem.com',           # remote domain
              'yosef',                   # remote id
              'secret',                  # remote password
              "access.log.".Yesterday(), # remote logfile input
              'c:/logs',                 # local dir to write to
              'access.log',              # local logfile output
              );
}
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but that is less useful for debugging. Its readability is a matter of what one's used to. Similarly, I might write Yesterday() as:

sub Yesterday {
    sprintf "%04d-%02d-%02d", Add_Delta_Days(Today(),-1);
}
[download]

If you want to really gild the lily, you could define a private namespace for your sub names: package myscript; sub Main{} sub Yesterday {} package main;. That would be really fussy though ;-)

After Compline,
Zaxo