sub how_many_lines
{
    open I, 'users.dat'
        or warn("Couldn't open users.dat\n"), return;

    # That single statment will try to open "users.dat"
    # (it better be in the current directory).  If that 
    # fails, it'll print "Couldn't open users.dat" and
    # return undef (the undefined value) to the calling
    # routine.  Yes, it's very terse, but that's part of
    # the power of Perl.

    my $lines;    # Declare $lines local to this scope (the sub)

    ++$lines while <I>;
    # Read the entire file a line at a time, incrementing $lines
    # every time.

    close I;      # Close the file handle

    $lines;       # return the count to the caller.

}