You don't specify what flavour of the dozens of Unices that are out there you are using, let alone which version. So, it's hard to solve the specific problem.

However, on my Linux box, "%s" prints the number of seconds since the Epoch. Since that's what gmtime is returning, why use the long about route using strftime in the first place? Wouldn't perl -lwe 'print gmtime' to the trick?