You can use the assign() function to assign a value to a name in an environment.
According to R Documentation:
assign(x, value, pos = -1, envir = as.environment(pos),
inherits = FALSE, immediate = TRUE)
A variable name, given as a character string. No coercion is done, and the first element of a character vector of length greater than one will be used, with a warning.
A value to be assigned to x.
Where to do the assignment. By default, assigns into the current environment.
The environment to use.
Should the enclosing frames of the environment be inspected?
An ignored compatibility feature.
In your case:
assign(paste("perf.a", "1", sep=""),5)