With that regexp you'll get an extra space at the end if the line is an even multiple of your breaking width. This handles that case:

my $width = 8;
my $break_at = 4;

my $orig = "#" x $width;

(my $broken = $orig) =~ s/(\S{$break_at})(?!\z)/$1 /g;

print qq{'$orig'\n}, qq{'$broken'\n};
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Result:

'########'
'#### ####'
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Update:

More ways to do it. With split:

my $broken = join( q{ }, grep { $_ ne q{} } split( qr/(\S{$broken_at})
+/, $orig ) );
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With substr (ugh):

my $broken = $orig;
for my $chunk ( 1 .. length($broken) / $break_at ) {
    substr( $broken, $break_at * $chunk + $chunk - 1, 0 ,
            $break_at * $chunk + $chunk - 1 == length($broken) ? q{} :
+ q{ }
    );
}
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