try

# in scalar context an array
# will return the number of its elements
print scalar @lines;

#which leads to
if (scalar @lines) {
  #array is not (an) empty (list)
  ...
}
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Regarding your question there is no need to worry if th earray gets newly populated each time, but if not in a block (especially loop) you might avoid such case by simply resetting the array with "@lines = ();".

Have a nice day
All decision is left to your taste