#!/usr/bin/perl
use v5.14;
use strict;
use warnings;

my $result  = 0;
my $level   = 2;		# next is F2
my $max_val = 4_000_000;
my $max_rec = 1000;

my $DBG = 1;


fib(1,0);			# F1,F0
print $result;


sub fib
{
    my  @frame = @_;

    #my $DBG = 1;

    unshift @frame, $frame[0] + $frame[1];   # prepend next level Fib (see footnote 1)
    pop @frame;                              # remove tailing Fib (optional)

    count($frame[0]);

    say "Fib(" . $level++ . ") =  $frame[0]" if $DBG;
    say " ... # \@frame = (@frame)" if $DBG;

    # playing safe with deep recursion    
    die "May recursion $max_rec exceeded" if $level > $max_rec; 

    return fib(@frame) unless($frame[0] >= $max_val);
}

sub count {
    my ($increment) = @_;

    #my $DBG = 1;
    
    if(($increment  % 2) == 0){
	say "adding Fib($level) = $increment to $result" if $DBG;
	$result += $increment;
    }
}