Indeed. Writing your own is more typing, and it's slower as well. About the only reason I can think of is wanting to avoid the loading of POSIX. But if that's significant, you probably want to inline your ceiling functionality anyway.

#!/usr/bin/perl

use strict;
use warnings;

use POSIX 'ceil';
use Benchmark 'cmpthese';

sub my_ceil {
    my $n = shift;
    int($n) + ($n > int($n)) 
}

our @tries = map {-100 + rand 50} 1 .. 1_000;

cmpthese -1, {
    ceil   => '@a = map {   ceil($_)} @tries',
    own    => '@b = map {my_ceil($_)} @tries',
    inline => '@c = map {int($_) + ($_ > int($_))} @tries',
};

__END__
        Rate    own   ceil inline
own    355/s     --   -52%   -54%
ceil   737/s   107%     --    -4%
inline 767/s   116%     4%     --
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