If you remove the extra parens (+ has a higher precedence than >), it seems to work:

$ perl -e '
use warnings;
use strict;

my @times = ( 1, 2, 3, 4);

print STDERR "Times: @times\n";
if (grep 0 + $_ >= 99999999, @times) { die("Time values must be < 9999
+9999"); }
'
Times: 1 2 3 4

$ perl -e '
use warnings;
use strict;

my @times = ( 1, 2, 3, 4);

print STDERR "Times: @times\n";
if (grep $_ + 0 >= 99999999, @times) { die("Time values must be < 9999
+9999"); }
'
Times: 1 2 3 4

$ perl -e '
use warnings;
use strict;

my @times = ( 1, 2, 3, 999999999999);

print STDERR "Times: @times\n";
if (grep $_ + 0 >= 99999999, @times) { die("Time values must be < 9999
+9999"); }
'
Times: 1 2 3 999999999999
Time values must be < 99999999 at -e line 8.
[download]

But I would advise you to use the block form of grep in such a case