# Give largesub and its entourage their own module
package ModuleWithLargeSub;

sub smallsub {
...process...
}

sub largesub {
    my ($e, $f, $g, $h) = @_;
    
    my $foo = &smallsub($e);    
    
    ...process...
    return \@AoH;
} 
   
1;

# Then in the cgi script:

#!/usr/bin/perl
use strict;
my $run_large_sub;
...etc...
...initiate variables...
...HTML::Template setup...

my $AoH;
if($run_large_sub) {
    require ModuleWithLargeSub;
    $AoH = ModuleWithLargeSub::largesub($a, $b, $c, $d);
}

$template -> param( todisplay => $AoH );
print $template->output();    
exit();