open OLD, $file or die "Can't open $file:$!\n";
open NEW, ">$file.new" or die "Can't open temp file $file.new: $!\n";
my $found =0;
while (<OLD>) {
	if (/^$username\t/) {
		$found =1;
		next; # skips to the next line, so won't print
			# any matching line to the new file
	}
	print NEW; # implicit "$_"
}
if ($found) {
	rename ($file, "$file.old") or die "Can't back up old $file: $!\n";
	rename ("$file.new", $file) or die "Can't rename new $file: $!\n";
} else {
	print "I didn't find $username!\n";
}