#! /usr/bin perl 

use strict;

use warnings;

...
...
# get session id
my session_id = get_cgisessionid();

...
...
sub a( $session_id );
...
...
sub b( $session_id );
...
...
sub c( $session_id );

..

sub a {
    my $session_id  = shift;
    ...
}

sub b {
    my $session_id  = shift;
    ...
}

sub c {
    my $session_id  = shift;
    ...
}