roboticus's example is a script that reverses itself (it opens $0), and it was run first, then printed, so it came out in reverse :-)

seek works in bytes, not characters or lines. You understood correctly that you can use it to move the current position around in the file (relative to its beginning, the current position, or its end). truncate cuts down the file to a certain size (AFAIK also in bytes, definitely not lines), but always from the beginning of the file.

I understand you want to preserve the first line of the file. You could do that by figuring out where that line ends (in bytes!), and truncate the file to there. However, since we're only talking about one line here, the logic would be much easier if you just clobber the entire file, modify the array of lines, and write everything back out.

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;
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But I would also second dasgar's suggestion for Tie::File.

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;
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Otherwise, I would not recommend opening files in read-and-write mode. But if you do want to do it this way, then you need to read the full file first and save the content in an array, make whatever changes you need to the array, and then only write back the array to the file. It works, but that would not be my recommended course of actions.

I would suggest reading the input file (read mode), writing to a copy of it (write mode), and then, if everything went OK, to do the house cleaning, i.e. deleting the old file (or archiving it under another name), and renaming the new one to what you need.

This is an important consideration iff the file will be accessed by other processes during the run of the script, or if it's important that the file doesn't get corrupted by either the script dieing or getting shot down externally while it is modifying the file. (probably what MidLifeXis's comment was aimed at as well)