local $/; # make undef
my @daily = do { local @ARGV = "daily.txt"; <> };
my @monthly = do { local @ARGV = "monthly.txt"; <> };
die "nothing to transfer" unless @daily > 3;
my @transfer = splice @daily, -3; # last three of @daily becomes @transfer
open OUT, ">daily.txt" or die "Cannot recreate daily.txt: $!";
print OUT @daily;
close OUT;
open OUT ">monthly.txt" or die "Cannot recreate monthly.txt: $!";
print OUT @transfer, @monthly;
close OUT;