This passes all both of your test cases:

use strict;
use warnings;

use Test::More tests => 4;

my $re = qr/(?:#(\S+))?/;

ok 'bli bla' =~ $re, 'No hash, matched';
is $1, undef, '$1 undef';

ok '#foo bli bla' =~ $re, 'With hash, matched';
is $1, 'foo', '$1 is foo';
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See also How to ask better questions using Test::More and sample data.