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;

##</code><code>##

use Tie::File;
my $MAXLINES=20; # not including header
tie my @lines, 'Tie::File', 'foo.txt' or die "tie failed";
splice @lines, 1, @lines-$MAXLINES if @lines>$MAXLINES;
push @lines, "newline\n";
untie @lines;