Tonight's TV highlights

Unreported World 7.30pm, Channel 4 After visiting a group of heroin-addicted men living in squalor under a bridge in Kabul, Ramita Navai uncovers a worse problem: a huge population of children, some as young as five, who are addicted to heroin. Many talk of getting hooked after losing parents to the war or witnessing suicide bombs, while supplies of extremely cheap heroin have exploded the situation. The access Navai gets is remarkable: she meets addicted families, children dependent on opium after taking it as a painkiller, and the doctors and social workers addressing the epidemic. WH New Tricks 9pm, BBC1 When messages claiming "I killed Flak" start to appear on walls in London, it's a cue for the plods to reinvestigate the death of a graffiti artist whose work now sells in galleries. No, this isn't a promising scenario, but happily we're not quite in Quincy punk rock episode territory here. Instead, things unravel as they usually do in New Tricks, which is another way of saying that Pullman has a wholly justified strop and Brian adds comic moments via his latest obsession, Twitter, and a new identity as @TopCop999. JW The Song Of Lunch 9pm, BBC2 Based on Christopher Reid 's award-winning narrative poem and starring Alan Rickman as a failed writer who meets up in Soho for lunch with old flame Emma Thompson , this is an unusual and highly satisfying departure for BBC drama, with a delicious emphasis on the verbal. Rickman's internal monologue is arrestingly rich – a waiter "slices into the infant conversation like a sweetly slung axe" – while also revealing him as a sad, stuffy, self-absorbed, lecherous old man, lost in a haze of bitter nostalgia and verbosity. DS MTV Hoods: Katy Perry 9pm, MTV The smut-popper may have been too rude for Sesame Street, but Katy Perry can, at least, handle a day in her home town of Santa Barbara, in what appears to be a beefed-up version of Cribs. She shows off her brother and sister, old friends and an old boyfriend, and it's a bit like reading a teenage diary, which is always more average and zitty than memory would suggest. In fact, there's not a lot to learn here apart from the amazing reveal that the woman who shot whipped cream out of her breasts in her last video has always been a bit of a show-off. RN Dexter 10pm, FX Dexter's bizarre friendship with the Trinity Killer (John Lithgow) continues to develop like most relationships – now they're going on an out-of-town "away break" together. Admittedly, their agendas may conflict with one another's (Trinity won't take too kindly to Dexter's plan to kill him; what, exactly is Trinity's "surprise" for Dexter?) but the pair's trip to Tampa makes for a fine episode. Ultimately, it's a question of which member of the Morgan family will get to Trinity first: Deb, with her powers of detection? Or Dexter, with a blunt instrument? JR Songwriters' Circle 10.15pm, BBC4 The dark art of songwriting is put under scrutiny by BBC4 this month, as a handful of masters of the craft do their best to uncover the secret to forging the perfect marriage of words and music, while performances by the great 70s-era songwriters are unearthed along the way. Here Chris Difford of Squeeze, Boo Hewerdine of the Bible and Justin Currie of Del Amitri share notes about writing their most famous songs, before performing them onstage together. WH