my $str = "This is a line # with comment";
my $word = '#';
while ($str =~ /[^\B#]($word)[^\B#]/g) {
    print "$1\n";
}
__OUTPUT__
#

##</code><code>##

my $str = "This is a line # with comment Boss.";
my $word = "#";

# define custom \b
my $b = qr/(?:(?=\S)(?<!\S)|(?!\S)(?<=\S))/;

# and match on non-space characters
while ($str =~ /$b(\S+)$b/g) {
    print "$1\n";
}

# or ignore the boundaries completely and match on non-space characters
while ($str =~ /(\S+)/g) {
    print "$1\n";
}