Part one of your question lost me (three times!) in (sic)

" ...and add a new along with a few other changes that I will mention. There aren't any other breaks in the file so as long as it found the FIRST, skipped down ONE line, printed a new ..then that would be fine....

Please, clarify!

However, the second part -- the one you characterize as "much more complicated" -- is not.

Others may well offer better approaches, but perhaps this long-winded but simple code (with lots of prints to help you track what's happening) will be useful. Note, however, that it's only a partial solution.

#!C:/perl/bin
use strict;
use warnings;

our $newdate = 0;
our $string = '<a href="SERVER122007/intranet.htm" target="_blank">Dec
+ 2007</a>';

$string =~ /.+?"SERVER(\d{6}).+blank">([A-Z,a-z]{3}\s\d{4}).*/;

our $ds = $1;
our $monyr = $2;  # changing this may require a dispatch table or a Da
+te::xxx module

print "\nRaw \$ds is $ds\n";
print "\$monyr is $monyr\nCode to modify this left as exercise (hint a
+bove) for the OP\n\n";

if ( $ds =~ /(\d\d).*/) {
   our $check_yr_change = $1;
   if ($check_yr_change == 12) {
      changeyear($ds);
   }
   else {
      print "OK! It's not yet time to change the year\n\n";
      $newdate = sprintf( '%.2x',(substr($ds,0,2) +1 ) ) . substr($ds,
+2,4);
      print "\$newdate is: $newdate";
   }
}

####### sub

sub changeyear {
   use vars qw($mo $oldyr $newyr);
   $mo = "01";
   if ( $ds =~ /\d\d(\d{4})/) {
      $oldyr = $1;
      print "\$oldyr: $oldyr\n";
   }

   $newyr = ( $oldyr + 1 );
   $newdate= ($mo . $newyr);
   print "\n\$newdate in sub is: $newdate\n";
}

=head OUTPUT
for <a href="SERVER072007/intranet.htm" target="_blank">Jul 2007</a>:

   Raw $ds is 072007
   $monyr is Jul 2007
   Code to modify this left as exercise for the OP

   OK! It's not yet time to change the year

   $newdate is: 082007

and for <a href="SERVER122007/intranet.htm" target="_blank">Dec 2007</
+a>:

   Raw $ds is 122007
   $monyr is Dec 2007
   Code to modify this left as exercise for the OP

   $oldyr: 2007

   $newdate in sub is: 012008
=cut
[download]

This would be somewhat shorter, were you to use Unix-ish (aka, pretty much standard, internationally) date formats; eg: YYYY-MM-DD. TTBOMK, the verbose form, "Jul 2007," is not a standard, but is common; in any case, incrementing it is less problematic.

Additional hint: Date::Manip is a fine "Leatherman" or "Swiss Army Knife." It handles an enormous range of date_time transformations and calculations, but I found it "not well suited" to this problem (or, maybe, I just didn't find the "well suited" functions) so it may repay you well to see if Date::Calc, Date::Parse and others in the Date::... family.