Why not:

my $filepath = 'foo.txt';

open my $FOOFILE, '>>', $filepath or die "Couldn't open $filepath: $!\
+n";
print $FOOFILE "First line of text.\n";

for (1 .. 5) {
    #do a bunch of stuff
    do_other_stuff ($FOOFILE);
}

print $FOOFILE "The end.";
close $FOOFILE;
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Unless there are multiple applications that may be updating the file and that are fighting for locks on it (in which case you are in a whole other world of pain) there is no reason to close the file. Opening and closing a file can be relatively expensive so avoiding that cost is generally a Good Thing™.

Note that you should avoid using global variables in subs ($FOOFILE for example) and generally you shouldn't call subs using the &sub syntax. Also, use the Perl for loop rather than the C for loop where you can.