Whenever I use a variable within contiguous text, I always delimit it. The '.' should terminate the variable name, because dots are not allowed in variable names. But the delimiter does help the human reader. And of course in situations where underscores are used in variables, you need to delimit to prevent accidents ( we frequently use date/time in file names, at work.)

Using the Perl rename command instead of shelling out is a superior solution. 99% of the time you think you need to shell out to use some standard Unix component, you need to look at the manual.

rename  'fpga_log.txt', $serial . '.txt';
# or maybe
rename  'fpga_log.txt', "${serial}.txt";
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