Lambda calculus! But it doesn't have integers (or any data types for that matter), you say? Sure it does, it has Church numerals!

my $two = sub { my($f,$x) = @_; $f->($f->($x)); };

sub add {
  my($m,$n) = @_;
  sub { my($f,$x) = @_; $m->($f, $n->($f, $x)) };
}

sub decode { my($n) = @_; $n->( sub{$_[0]+1}, 0 ); }

print decode( add($two, $two) ), $/;
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Yeah, at some point we have to convert the Church numeral to something that Perl can print.. but you get the idea.