Are you trying to modify an existing crontab to run your script, or are you trying to get your script to act as if it is the scheduler?

If you want to modify the user's crontab, you might be able to do something like this:

open my $ct_in, '-|', 'crontab -l' or die "Can't read crontab: $!";
my $crontab_contents = do { undef $/; <$ct_in> };
close $ct_in or die "Can't close crontab after read: $!";

open my $ct_out, '|-' 'crontab -' or die "Can't write new crontab: $!"
print $ct_out $crontab_contents;
print $ct_out "\n";
print $ct_out "*/5 * * * * /every/five/minutes\n";
close $ct_out or die "Can't close crontab after write: $!";
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You can get the format of the crontab file from man 5 crontab.