I watched an MIT lecture on youtube demonstrating some of the abstraction power of LISP. Then I tried my hand at duplicating the functionality in Perl. Below I will post the LISP code, and the Perl equivalent.

LISP

(DEFINE (SUM TERM A NEXT B)
        (IF (> A B)
                0
                (+ (TERM A)
                        (SUM TERM
                                (NEXT A)
                                NEXT
                                B))))

(DEFINE (SUM-INT A B)
        (DEFINE (IDENTITY X) X)
        (SUM INDENTITY A 1+ B))

(DEFINE (SUM-SQ A B)
        (SUM SQUARE A 1+ B))

(DEFINE (SQUARE A) (* A A))


(SUM-INT 3 5)

(SUM-SQ 3 5)
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Perl

#!/usr/bin/perl
use warnings;
use strict;
use feature "state"; 

sub sum # TERM A NEXT B
{    
    state $sum;
    my ($TERM, $A, $NEXT, $B) = @_;
    if ($A>$B){ 
        my $temp=$sum;
        $sum=0;
        return $temp ;
        }
    $sum+=&{$TERM}($A);
    sum( $TERM, &{$NEXT}($A) ,$NEXT, $B);
}

sub identity{
    shift;
    }

sub square{
    ((shift)**2);
    }

sub sumint { sum(\&identity,shift,sub {((shift)+1)},shift)}

sub sumsq { sum(\&square,shift,sub {((shift)+1)},shift)}

#a normal sum from 3-5
print sumint(3,5) . "\n";
#a sum of squares from 3-5
print sumsq(3,5) . "\n";
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