Use another hash to hold the variables and just keep adding your new hash to it. Anything you change will override the previous values, anything you don't will just stay there.

#!/usr/bin/perl5.6
use Text::Template;
my $template = Text::Template->new(SOURCE => 'formletter.tmpl')
          or die "Couldn't construct template: $Text::Template::ERROR"
+;
my @monthname = qw(January February March April May June
                           July August September October November Dece
+mber);
my %vars1 = (title => 'Mr.',
                    last_paid_month => 1,   # February
                    amount => 392.12,
                    monthname => \@monthname,
                   );

my %keepvars = %vars1;

my $result = $template->fill_in(HASH => \%keepvars);
if (defined $result) { print $result }
else { die "Couldn't fill in template: $Text::Template::ERROR" }
my %vars2 = (title => 'Mr.',
                    firstname => 'Bill',
                    lastname => 'Gates',
                   );

%keepvars = (%keepvars, %vars2);

$result = $template->fill_in(HASH => \%keepvars);
if (defined $result) { print $result }
else { die "Couldn't fill in template: $Text::Template::ERROR" }
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