Strange that nobody has mentioned truncate. This would get rid of the overhead of rewriting the whole file.

#!/usr/bin/perl
#
# given a file name as an argument,
# read the file until eof, back up
# one line and truncate the file there.
# in short, "delete the last line of a file"
#
my $file=shift(@ARGV);
open(FILE,"+<$file") or die;
while (<FILE>) {
    if (eof(FILE)) {
       seek(FILE,-(length($_)),2) or die;
       truncate(FILE,tell(FILE)) or die;
    }
}
close FILE;
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This would be faster for large files if it would seek() to eof, then read backwards looking for the newline. Seemed a bit cumbersome for an example though.