#!/usr/bin/perl
use strict; use warnings; use feature qw(say);
sub sum_from_to{
    my ($TERM, $from, $NEXT, $to)=@_; 
    if( $from > $to ){
        0; 
    } else {
        $TERM->($from)
        +   
        sum_from_to(
            $TERM,
            $NEXT->($from),
            $NEXT,
            $to
        );  
    }   
}
sub addone{ return 1 + $_[0]}
sub self{ return $_[0]}
sub square{ return $_[0] * $_[0]}

sub sum_int {
    my($A,$B)=@_;
    sum_from_to( \&self, $A, \&addone, $B);
}   
sub sum_sq {
    my($A,$B)=@_;
    sum_from_to( \&square, $A, \&addone, $B);
}  

say sum_int(3,5);
say sum_sq(3,5);