assembling a signed integer from bytes

[Steve]

Hi all

I am reading data from a serial device with perl, capturing each group
as an array of numbers generated from the ord of each character. I
don't know exactly how it is formatted, but having looked at the way
the values vary and knowing the type of data being transmitted, it
seems likely that I have several signed numbers each compsed of two of
my bytes.

How do I assemble two variables, each carrying a number 0-255 into a
single, two-byte signed number?

Thanks

Simon