use POSIX qw( ceil );
my $rounded = ceil($num*2)/2;
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Update: Oops, the above didn't work for negative numbers.

use POSIX qw( ceil );
my $rounded = ($num>=0 ? +1 : -1) * ceil(abs($num)*2)/2;
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or

use POSIX qw( ceil floor );
my $rounded = ($num >= 0
   ? ceil($num*2)/2
   : floor($num*2)/2
);
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