$ perl -E '
    my $x = qq{ A B\tC # comment};

    say q{Original string: |}, $x, q{|};

    say q{s/\s//g:         |}, $x =~ s/\s//gr, q{|};
    say q{s/\s//gx:        |}, $x =~ s/\s//grx, q{|};
    say q{s/\s//gxx:       |}, $x =~ s/\s//grxx, q{|};

    say q{s/ //g:          |}, $x =~ s/ //gr, q{|};
    say q{s/ //gx:         |}, $x =~ s/ //grx, q{|};
    say q{s/ //gxx:        |}, $x =~ s/ //grxx, q{|};

    say q{s/[ ]//g:        |}, $x =~ s/[ ]//gr, q{|};
    say q{s/[ ]//gx:       |}, $x =~ s/[ ]//grx, q{|};
    #say q{s/[ ]//gxx:      |}, $x =~ s/[ ]//grxx, q{|};
    say q{s/[\ ]//gxx:     |}, $x =~ s/[\ ]//grxx, q{|};

    say q{s/#//g:          |}, $x =~ s/#//gr, q{|};
    say q{s/#//gx:         |}, $x =~ s/#//grx, q{|};
    say q{s/#//gxx:        |}, $x =~ s/#//grxx, q{|};

    say q{s/[ #]//g:       |}, $x =~ s/[ #]//gr, q{|};
    say q{s/[ #]//gx:      |}, $x =~ s/[ #]//grx, q{|};
    say q{s/[ #]//gxx:     |}, $x =~ s/[ #]//grxx, q{|};
'
Original string: | A B  C # comment|
s/\s//g:         |ABC#comment|
s/\s//gx:        |ABC#comment|
s/\s//gxx:       |ABC#comment|
s/ //g:          |AB    C#comment|
s/ //gx:         | A B  C # comment|
s/ //gxx:        | A B  C # comment|
s/[ ]//g:        |AB    C#comment|
s/[ ]//gx:       |AB    C#comment|
s/[\ ]//gxx:     |AB    C#comment|
s/#//g:          | A B  C  comment|
s/#//gx:         | A B  C # comment|
s/#//gxx:        | A B  C # comment|
s/[ #]//g:       |AB    Ccomment|
s/[ #]//gx:      |AB    Ccomment|
s/[ #]//gxx:     | A B  C  comment|

##</code><code>##

Unmatched [ in regex; marked by <-- HERE in m/[ <-- HERE  ]/ at -e line 16.

##</code><code>##

my $re = qr{(?x:
    ... multiline regex pattern here ...
)};