use warnings;
use strict;

my $str = 'a|b|c d    e  ';
$str =~ s/[\s|]/_/g;
print "$str\n";

$str = "w\nx\ny\nz";
$str =~ s/\n/ /g;
print "$str\n";

__END__

a_b_c_d____e__
w x y z

##</code><code>##

The regular expression:

(?-imsx:\s|\(|\))

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \s                       whitespace (\n, \r, \t, \f, and " ")
----------------------------------------------------------------------
 |                        OR
----------------------------------------------------------------------
  \(                       '('
----------------------------------------------------------------------
 |                        OR
----------------------------------------------------------------------
  \)                       ')'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------