I read a little about the workings of the timer, and I realized my problem was that I enabled the timer 0 interrupt, while I just needed to enable the external one, configure the timer and make a loop for the time needed. The header isn't really needed, because Paulmon can access programs with a JUMP function.

Here is the code that worked (the normal loop was from one of PJRC examples)

.equ    locat, 0x2000          	

.equ    port_e, 0xF901		
.equ    port_def_pgm, 0xF903	

.org	locat			
	lcall	start		

.org    locat + 3		
timer0_isr:			
	push	psw		
	push	acc		
	push	dpl		
	push	dph
	mov	dptr, #port_e	
        lcall 	cicle		
	pop	dph		
	pop	dpl
	pop	acc		
	pop	psw     	
	reti			

.org 	locat + 48		
start:
	mov	dptr, #port_def_pgm
	mov	a, #128		 
	movx	@dptr, a	

	mov	tmod, #0x11	
				
	mov	ie, #0x81	

loop:
	clr	it0		
[...]	

;=======================================================
;                 Timer Configuration
;=======================================================

cicle:				
	clr	tr0		
	clr	tf0		
	anl	tmod, #0xF0	
	orl	tmod, #0x01	
	mov	tl0, #0
	mov	th0, #0		
	mov	r2, #56		
	setb	tr0		
	sjmp	loop1		
loop1:
	mov	a, #0x55	
	movx	@dptr, a
	jnb	tf0, loop1	
	clr	tf0		
	djnz	r2, loop1	
	mov	r2, #56		
	sjmp	loop2		
loop2:
	mov	a, #0xAA	
	movx	@dptr, a
	jnb	tf0, loop2	
	clr	tf0		
	djnz	r2, loop2	
	mov	r2, #56		
	sjmp	loop3		
loop3:
	mov	a, #0x55	
	movx	@dptr, a
	jnb	tf0, loop3	
	clr	tf0		
	djnz	r2, loop3	
	mov	r2, #56		
	sjmp	loop4		
loop4:
	mov	a, #0xAA	
	movx	@dptr, a
	jnb	tf0, loop4	
	clr	tf0		
	djnz	r2, loop4	
	mov	r2, #56		
	sjmp	loop5		
loop5:
	mov	a, #0x55	
	movx	@dptr, a
	jnb	tf0, loop5	
	clr	tf0		
	djnz	r2, loop5	
	ret