my $MAXLINES=20; # not including header
open my $fh, '+<', 'foo.txt' or die $!;
my @lines = <$fh>;
splice @lines, 1, @lines-$MAXLINES if @lines>$MAXLINES;
push @lines, "newline\n";
truncate $fh, 0 or die "truncate failed";
seek $fh, 0, 0 or die "seek failed";
print $fh @lines;
close $fh;