#!/usr/bin/perl --
use strict; use warnings;
use Devel::TraceCalls { Subs => [qw/fib /]};
print fib(5);
sub fib {
  my $n = shift;
  if ($n < 2) { return $n }
  fib($n-2) + fib($n-1);
}
__END__
TRACE: main::fib( 5 )
TRACE: +-main::fib( 3 )
TRACE: | +-main::fib( 1 )
TRACE: | +-main::fib( 2 )
TRACE: | ! +-main::fib( 0 )
TRACE: | ! +-main::fib( 1 )
TRACE: +-main::fib( 4 )
TRACE: | +-main::fib( 2 )
TRACE: | ! +-main::fib( 0 )
TRACE: | ! +-main::fib( 1 )
TRACE: | +-main::fib( 3 )
TRACE: | ! +-main::fib( 1 )
TRACE: | ! +-main::fib( 2 )
TRACE: | ! | +-main::fib( 0 )
TRACE: | ! | +-main::fib( 1 )
5