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

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

key value
f1  f2
...
f3
f4  f5
f5

my %calls; #has all the function call information
my $length = 3; #the length of function chains
...
    }
  }
}