Indomitus beat me to the submit button, I see.

if($pword eq $cpword){
   open (FILE, "< memberlist.txt") or die $!;
    
   while(<FILE>){
      push @lines,$_;
   }
   close FILE or die $!;
}
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• You'll be able to zero in on problems like this if you habitually use the die statement, to test whether you've had success.
• Unless your passwords are numeric, you'll want to test equivalence with 'eq', unstead of '=='.
• And finally - your code closed the filehandle inside of your while loop (another thing that die would have told you).