Alternatively, you can have Perl leave behind a backup file
perl -i.bak -F: -lane 'print unless $F[0] eq "billg"' .users
This is the same as -i
Except Perl does not remove the original file
Instead, it renames it to file.bak
If Perl crashes partway through, the old data is still available in file.bak
(Or if you decide you don't like the change.)