You should have a <FILE> in the while() loop. The <FILE> tells perl to loop through your file one line at a time. You can also read the whole file into memory by doing @lines = <FILE> but it's up to you if you want to use that memory.

if($pword==$cpword){
    
    open (FILE, "memberlist.txt");
    
    while(<FILE>){
    push @lines,$_;
    
    close FILE;
}    
}
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UPDATE: You might also want to try checking your open statement to make sure it worked.

open (FILE, "memberlist.txt") || die "Cannot open: $!";
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The $! gives you the error message from the open command if it failed.