f1 calls (f2)
f2 calls (f3,f4)
f3 calls nothing
f4 calls f5
f5 calls nothing

##</code><code>##

f1->f2->f3
f1->f2->f4
f2->f4->f5

##</code><code>##

key value
f1  f2
f2  f3,f4
f3
f4  f5
f5

##</code><code>##

my %calls; #has all the function call information
my $length = 3; #the length of function chains
my @chain_store; #array of array, to save the chains
while (($key, $value) = each %calls) {
 my @chain = ();
 push @chain, $key;
 get_chains($value, \@chain);
}

sub get_chains {
  my $value = shift;
  my $chain_ref = shift;
  my @chain = @{$chain_ref};
  foreach my $val @{$value} {
    push @chain, $val;
    my $is_present = 1;
    for (my $count = 0; $count < $length; $count++) {
      if (exist($calls{$val})) {
        get_chains($calls{$val})
      } else {
        $is_present = 0; 
        last;
      }
    }
    if ($is_present) {
       push @chain_store, @chain;
    }
  }
}