Date::Manip should do what you want:

#!/usr/bin/perl

use strict;
use warnings;

use Date::Manip;

my @dates;

while( <DATA> ) {
  chomp;
  push( @dates, ParseDate( $_ ) );
}

@dates = sort @dates;
my $oldest = $dates[0];

foreach my $current ( @dates ) {
  my $diff = DateCalc( $oldest, $current );
  print "Diff between ", 
        UnixDate( $oldest, "%m/%d/%y" ), 
        " and ", 
        UnixDate( $current, "%m/%d/%y" ), 
        " is ", 
        Delta_Format( $diff, "approx", 0, "%Mt" ),
        "\n";
}

__DATA__
06/30/96
08/07/97
09/05/97
12/11/97
03/26/98
07/17/98
10/23/98
11/12/98
01/14/99
04/10/99
05/27/99
02/10/00
03/10/00
03/15/00
03/15/00
03/30/00
06/15/00
07/13/00
08/03/00
09/07/00
09/30/00
12/10/00
01/25/01
02/27/01
[download]

Update: If the list is all ready sorted (and you need a bit of golf)

#!/usr/bin/perl

use strict;
use warnings;

use Date::Manip;
use List::Util qw( reduce );

my @dates = qw( 06/30/96 08/07/97 09/05/97 12/11/97 03/26/98
                07/17/98 10/23/98 11/12/98 01/14/99 04/10/99
                05/27/99 02/10/00 03/10/00 03/15/00 03/15/00
                03/30/00 06/15/00 07/13/00 08/03/00 09/07/00
                09/30/00 12/10/00 01/25/01 02/27/01 );

reduce {
  print "Diff between $a and $b is ", Delta_Format( DateCalc( $a, $b )
+, "approx", 0, "%Mt" ), "\n";
  $a;
} @dates;
[download]