Pinky and Perky are singing puppet piglet twins. Dressed alike, either in sailor suits, or blue dungarees with yellow tops, Perky always has the hat. Their voices were created by speeding up speech, creating a squeaky, trill tone.

They were conceived in 1956 by pupetteers Jan and Vlasta Dalibor, a Czechoslovakian couple living in Britain. The Dalibors wanted to create simple characters that would be loved by children: hence their singing and dancing.

Between 1957 and 1973, Pinky and Perky enjoyed their own TV show, featuring both hand- and string-puppets. The show was a mixture of chat, sketches and music, and attracted some very well-known celebrities, such as The Beatles, and Harry Secombe.

They also featured in many live shows, including the London Palladium, and Las Vegas.

In addition to their appearances on TV and on the stage, they also released 16 albums.

The squeaky singing animal phenomenon can be seen in other countries' popular culture:

America - Alvin and the chipmunks.
Finland - Pikkuoravat (The Tiny Squirrels)

Sources:
http://www.pinkyandperkypop.com/html/history/index.html
Noded while listening to Pikkuoravat for inspiration!