Will this do what you want?

my (@memberlist, @allmembers);

foreach my $member (@memberlist) {
   open(FILE, "$member.dat")  or
      die "Open failed for file: '$member.dat'. $!\n";
   lock(FILE);
   my @members = <FILE>;
   unlock(FILE);
   close(FILE);
   chomp @members;
   push (@allmembers, join (" ", @members));
}
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Update: tilly is absolutely right! We should know better.

"Make everything as simple as possible, but not simpler." -- Albert Einstein