my $pcname = 'pee-cee-name';
my $log = "\\\\$pcname\\c\$\\windows\\windowsupdate.log";
print "$log\n";
__END__
\\pee-cee-name\c$\windows\windowsupdate.log
[download]

Inside quotes, a backslash is an escape character. To get a literal backslash, you need two of them, and you need one before the dollar sign so it's not introducing a variable.