@bits = ( $scalar =~ /.{1,$bytes}/gs );
[download]

Edited by tye to add "s" on the end otherwise "." won't match newlines.

Edited by chipmunk to change {0,$bytes} to {1,$bytes}; otherwise a null string is matched at the end.

push @bits, substr $scalar, 0, $bytes, '' while length $scalar;
[download]

If by "bytes" you mean ASCII characters from a string, this is a simple and relatively fast solution:

my $string = "1234567890abcdefghijABCDEFGHIJK";
my $n = 2;    # $n is group size.
my @groups = unpack "a$n" x (length( $string ) /$n ), $string;
[download]

This solution has a couple of nice bonuses. First, it will only return complete groups. If you have extra characters that don't complete out a group, it won't give them to you. Second, it is easily adaptable to any other data type that pack and unpack support.

UPDATE: As Aristotle pointed out here, if you want the last group even if it's incomplete you may do this:

my @groups = unpack "a$n" x ((length($string)/$n)-1) . "a*", $string;
[download]